A free client that enables conversation with buddies who use AOL Instant Messenger or just plain AOL. Because of a disparity in the protocol used, older versions of AOL's main software (before 6.0) did not have as large of a feature set as the free Instant Messenger downloadable client.
The Windows and Mac OS clients offer Direct IM, where your messages are not sent through AOL's servers but rather directly, Voice Chat using a microphone, group chat, file transfer, and of course a lot of advertisements everywhere. AOL's quasi-official Linux client, as well as many clones, offers fewer features but also does not have ads. The TOC protocol, which is "open," allows some basic features like group chat and server-based messaging, while the closed OSCAR protocol offers the full suite.
Follow-up: On March 16, I was using Everybuddy over VNC when a friend asked me to send him a file. Of course, such an advanced feature isn't supported in the TOC client, so I decided to use the AIM client for Macintosh, installed on my workstation. I started it up, and logged myself in. I was treated to a small, albeit resizable, window with an ad banner, a tab control, a list control with a total of six "buddies" visible, five command buttons, a search box, a help button, another ad banner, a stock ticker, and a status line. Other windows popped up, too: first a mail notification window, then a news ticker, and finally a "Welcome to AOL Instant Messenger" window. I know that much of this can be disabled, but it seems like it would be really scary to a new user to be assaulted with all of this eye candy.
The scary part is that other instant messaging programs are trying to join the "all-in-one" craze: Yahoo! Messenger offers a dizzying array of add-ons; MSN Messenger lets you access content from many sites by ".NET My Services", formerly Hailstorm; and Trillian has just released a commercial product called Trillian Pro which adds external media content and many more features. Yahoo! Messenger and MSN Messenger are free, as is AOL's read-headed stepsister ICQ. Trillian Pro costs $25.