Tie a string around your finger. Write a message on a Post-it note. Leave yourself a voice mail. For centuries, people have come up with ingenious ways to aid their memories. But it’s still all too easy to forget. A startup called ReQall, based in Moffett Field, CA, today launches a service designed to jog users’ memories depending on what they’re doing at any given moment.

ReQall already offers a free service that can be accessed using a phone. By calling a toll-free number, users can record a memo, reminder, or appointment, and voice-recognition software will analyze each message, turning it into the appropriate kind of note. The service issues timely reminders via IM or e-mail and a daily summary of appointments. Users can also access and modify their memos online.

A new version of the software called ReQall Pro, which launches today, focuses on issuing reminders in the right place at the right time. ReQall Pro works with iPhones and Blackberrys, using the location information collected by these devices, as well as information gleaned from the contents of each memo, to work out when (and where) to jog a user’s memory. “We believe that computers can help with everyday memory problems and help with organizing,” says Sunil Vemuri, cofounder of ReQall.

ReQall Pro’s “memory jogger” software determines how to issue reminders to users. It performs keyword analysis on memos that a user enters in an effort to link together relevant information. For example, if a meeting with John Doe is approaching, ReQall will present the user with other stored items related to John Doe. A user will also automatically receive reminders when he arrives at certain locations, for example, receiving a grocery list when he reaches the grocery store. Vemuri explains that the system also tries to avoid overloading the user by paying attention to how many notes a user has stored and optimizing the number of reminders issued. It will also adjust to a user’s behavior, issuing a limited number of reminders at locations that a user visits frequently.

Vemuri got the idea for ReQall after doing research for his PhD, which involved recording everything about his life for several years. “I would not advise doing that anymore,” he jokes. “There’s too much bathwater and too few babies in there.” Since then, he has focused instead on helping users store important information more easily, and figuring out how best to filter it.

Michael King, a research director at Gartner specializing in wireless technology, says that he’s impressed by ReQall’s focus on context. “There’s nothing that I’ve really seen out there that takes a bunch of these different aspects of context and melds it into a single application,” King says. He adds that, while ReQall’s service is impressive, assistants of this type will be most useful when they can go even further. For example, instead of reminding a user to purchase tickets, the application might handle the purchase itself.

In addition to the memory jogger technology, ReQall Pro includes integration with Outlook and Google Calendar. The Pro service costs $2.99 a month, or $24.99 a year. Existing users will be able to purchase the service at a discounted rate. ReQall Standard will continue as a free service, but may include advertising in the future.