Without Google, Yahoo, and their brethren, using the Web would be like wandering through a library where a prankster has restacked the books at random. Still, sorting through thousands or millions of pages spurted out by a search engine can be nearly as overwhelming. Imagine instead being able to call on the group judgment of other users, people who are constantly skimming the latest Web content and arranging the best stuff into neatly labeled piles.

It’s called tagging, and it’s going on at a handful of free websites – Del.icio.us, Flickr, Furl, and Rojo, among others – where members are voluntarily classifying and categorizing thousands of pieces of content each day. The phenomenon is growing fast: Delicious alone had 90,000 total users by April 2005, up from 30,000 the previous December.

Taggers aren’t acting unselfishly. The main point of these sites is to allow members to store the addresses of Web pages that interest them, which they can later revisit from any computer or browser. Along the way, they are invited to add tags to the pages they bookmark – descriptive terms of their own choosing that make it easier to find favorite sites in a big stack of bookmarks. But there’s a twist that makes tagging a collectivist undertaking: everybody can see everybody else’s tags. The ability to see what other people are tagging can lead you to material you might not discover on your own. “It represents a break in the traditional assumptions about how we know things,” says David Weinberger, the author of Small Pieces Loosely Joined: A Unified Theory of the Web and the coauthor of The Cluetrain Manifesto, an e-commerce classic published in 2000. Tagging is “not a structure that’s given to us. It’s one that is created by the user.”

Tagging is already attracting the attention of the traditional search industry. In March, Yahoo acquired Flickr, a photo-sharing site where users can tag their own pictures and others’. Technorati, a site that tracks the most-discussed subjects on Web logs (or “blogs”), makes extensive use of tags from Del.icio.us, Flickr, and Furl. Last September, Looksmart purchased Furl, and it is now adding the ability to create instant Furl bookmarks to its family of specialized search sites. “It makes a lot of sense to have a capability that allows for sharing of essential Web pages,” says Debby Richman, Looksmart’s senior vice president of consumer product development.

There’s no uniformity to the way people tag Web pages, so the same tag might wind up being applied to very different kinds of content. But to most developers, that’s actually a strength of the technology, not a weakness. “The information you get [through tags] is always going to be somewhat imperfect and fuzzy,” says Joshua Schachter, the creator of Del.icio.us. “But a bunch of people doing ‘okay’ tagging may actually have a higher net value than an authoritative organization telling you how information should be organized.”