I see many on here spouting the virtues of Trillian, so i thought this might be of interest:

People using the popular Trillian software may lose access to part of AOL Time Warner's instant messaging system.
Firing another salvo in the battle over instant messaging, America Online is blocking people using the Trillian interface from linking to its AOL Instant Messenger (AIM) system.

Trillian, which allows people to access multiple instant messaging programs from one screen, has attracted nearly a million people who are hoping to streamline their use of incompatible systems, including AOL's AIM and ICQ, Yahoo's Messenger and Microsoft's MSN Messenger.

But in the past 24 hours, an elaborate game of cat and mouse has developed between AOL and Trillian creator Cerulean Studios--as the start-up has repeatedly released new software designed to get around the block, prompting AOL to rush in and stop people from using it.

Meanwhile, Trillian fans are snared in cross fire.

"AOL is being selfish," Trillian user Mike Cicciarelli said in an instant messaging exchange with News.com.

Trillian user Chris Hilbert said he's downloaded new software several times in the past day in an attempt to re-establish access with AIM. Hilbert said he uses Trillian because it's simple.

"I like that there are no ads, less bloat," said Hilbert, an Indiana University student and creator of the FileFlash Web site. "I don't have to have four different clients open."

But AOL said it's a matter of security and is framing the conflict as yet another chapter in the instant messaging wars. In recent years, AOL has clashed with other makers of IM software, including Microsoft and AT&T. Both companies tried to create systems that let people correspond with AIM users--only to have them blocked.

"It has long been our very public policy that when a service unleashes software that hacks into our system, and endangers the security of our system, we stop it," AOL spokeswoman Kathy McKiernan said.

McKiernan said that Trillian does not have a business relationship with AOL. "To the extent that consumers think they do, they were misled," McKiernan said.

Meanwhile, the creators of Trillian plan to keep up the tit-for-tat efforts. Since AOL first started blocking the software Tuesday, they have released one workaround and at least two more full downloads of Trillian.

Michael Gartenberg, a research director at Jupiter Media Metrix, called AOL's practice "predatory," but he said at least the media behemoth is consistent in blocking both large and small companies from accessing its system.

"There are no technical reasons they can't open up IM, but there are a lot of business reasons not to," Gartenberg said. "They're not going to unless they're forced to."

News.com's Evan Hansen contributed to this report.