Gmail is the best thing that ever happened to Sun Microsystems.

Of course, Googles Gmail service doesnt directly have any relationship with Sunat least none that Google has disclosed. Like most of Googles infrastructure, Googles Gmail service is probably hosted on racks and racks of Intel-based computers running some variant of Linux. Googles expertise is in making thousands of these machines run in concert as a single computational resourcea resource that can store thousands of terabytes and satisfy simultaneous requests from millions of users.

Sun, on the other hand, makes much of its money these days by selling big iron computers to companies that havent mastered the art of so-called grid computing. Suns customers find it easier to scale their computing problems by throwing money at their problems. It sells mainframe-class machines with dozens or even hundreds of processors that are incredibly reliable and capable of handling tens of thousands of simultaneous requests. The economics behind what Google is doing and what Sun is selling couldnt be more different.

Nevertheless, Gmail validates a claim that Sun has been making for nearly a decadethat its possible to replace a network of PCs running Windows with world-class computers offering computing services to low-cost and easily-managed desktop machinesperhaps machines so inexpensive that they dont even have a hard disk. Sun called such computers thin clients. While they are popular at some companies, they havent made real inroads against the Windows desktop because the applications just havent worked as well.

But Gmail does work just as well as a copy of Outlook Express running on the desktop. In some way, in fact, it works better. This is big newsbigger, in fact, then most people seem to realize.

Until Gmail, practically every Web-based application was a pale imitation of that same application running on a PC. Web-based applications had the advantage that they were accessible from any computer on the Internet on professionally managed servers, that the data was backed up, and that the applications themselves were constantly updated. But compared to applications running on your local machine the web versions had fewer features and performed more slowly. Most usersbusinesses and consumers alikewere unwilling to make that compromise.

Gmail is different. For starters, its blindingly fastso fast that it feels like it is running on your local computer and not in some data center. Click on a messages subject and it instantly appears. When you are done reading a message you click Archivethe message is instantly stored, and youre looking back at your inbox. (As with other Web-based mail systems, you can report spam simply by clicking report spam.)

Gmail gets its speed from some of the cleverest JavaScript ever written. Lots of information is stored inside your browser and redisplayed from memory; this avoids the need to constantly download pages from Googles servers. The JavaScript can “listen” directly to your keystrokes, allowing you to drive Gmail with single-letter commands: press y to archive a message, c to compose a new message, and so on. You dont even have to depress the Ctrl key, making Gmail even faster to use.

Gmail shows that Web applications with thin clients can have advantages over software running on your desktop. The most obvious is reliability: Gmail runs on Googles servers, not your hard drive, and Google almost certainly does a better job than you do with routine maintenance, backups, and the like. And because everything is kept on Googles servers, you dont have to wait for long downloads. Googles computers are blazingly fast: searching through the few thousand messages stored in my Gmail account is essentially instantaneous. Searching through the same amount of mail on my local computer takes ten seconds or more.

Gmails anti-spam system is nothing short of phenomenal. I sent Gmail a copy of my entire inbox for two weeksthats 200 real messages a day plus 500 pieces of spam. My anti-spam system at home let through about 20 spams a day; Gmail let through fewer than 5. Gmails big advantage in the anti-spam department is its ability to harness the collective vigilance of all Gmail users. Once a message has been reported as spam by a few dozen users, Gmails servers can pull that message out of everybody elses inbox.

There are problems with Gmail, of course. Im one of 31 privacy advocates and civil liberties organizations to have signed an open letter calling upon Google to suspend Gmail until the services privacy issues are properly addressed. One concern raised in the letter is that Google scans all incoming e-mail for the purpose of displaying targeted advertisements. I think that the growing trend toward putting ads in application software is troublesome, no matter whether the applications are running on my computer or on a remote server. Right now the only way Google makes money on Gmail is by having people click on those ads, but I fully expect a subscription-based version of Gmail to be available within a year that allows me to make those ads go away by paying a monthly fee.

A bigger problem with Gmail is the companys data management and retention policy. I like to think that I own the e-mail that I receive, but with Gmail Im definitely sharing that ownership with Google. If I decide to cancel my account, there’s no obvious way to download all of my mail thats stored on Gmail. One of my annual housecleaning chores is to make a CD-ROM with all of the past years e-mail, but Google does not provide the tools that would let me do that, short downloading each Web page to a separate file. (Apparently, however, independent developers are filling the void that Google has left; click here, for example.) Unfortunately, use of such software appears to violate the Gmail terms-of-service agreement, which forbids the user from using “any robot, spider, other automated device, or manual process to monitor or copy any content from the [Gmail] Service.” What’s more, even if I terminate my Gmail account, Google essentially reserves the right to keep my stored email forever. (Here’s the complete privacy policy.) Clearly, with possession of data comes control over that data.

Theres also concern that Gmail might become a one-stop shopping service for law enforcement, much in the way that America Online has become. Having a gigabyte of stored e-mail makes Gmail an attractive target: its not just a window into your present activities, but into your past as well. Just imagine the potential in a divorce proceeding!

Like any other Web-based application, Gmail doesnt work if youre not connected to the Internet. Now, e-mail might seem like an inherently network-based application, but I frequently download e-mail to my laptop and then read it while Im off the networkfor example, when Im in a room that doesnt have wireless connectivity or when Im flying off to California.

One problem that I have with Gmail is its incompatibility with other e-mail systems: Gmail does not support either POP or the increasingly popular IMAP mail protocol. As a result, I was unable to use Gmail with my Treo 600 using Sprints wireless network. Google has said that it intends to support mobile devices directly before the end of the year. But the lack of IMAP support means that I cant easily transfer e-mail messages to or from Gmail the way I can between other IMAP-based services.

As my last note of singing Gmails praises, I need to point out that it seems to work equally well with practically every other browser that Ive been able to throw at it, including Internet Explorer for Windows, Apple’s Safari for MacOS 10, and Mozilla Firefox for both. This is no easy feat for an application this sophisticated in its use of JavaScript. Google has clearly gone out of its way to show that complex Web-based applications can be developed and deployed without relying on all of that Microsoft-specific junk thats been crammed into IE. Other websites should take notice.

Many companies will probably decide that Gmail is not appropriate for business use, since businesses typically want to maintain control over their own e-mail. Perhaps Google will bring out a version of Gmail running on an appliance, the way the company now sells a search appliance. But the idea of search-based e-mail is catching on. Bloomba, for example, is an e-mail client that offers many of the searching, spam protection, and automatic message filing that Gmail hasbut it works with any POP or IMAP-based account. As with Gmail, you just click a file button when you are done reading a message with Bloomba and the message is filed; you get your filed messages back by using search.

Gmail is going to make a big impact, and e-mail will never be the same. But Gmail also proves an argument that Suns been making for more than a decade. I wonder if they will exploit it.