On Windows and Linux, Pidgin is my IM client of choice. On Mac I use Adium, which is sort of related to it since they share some common libraries under the hood, even though there is a Pidgin client for OS X. I'm optimistic that the latest release might make it easier to use voice and/or video chat online. I don't do that often enough because its usually a headache, even with iChat. iChat used to work fine but with Verizon's FIOS router, I have to put my computer in the DMZ outside the router's firewall to get it to work. Look for it to make it into your linux distro soon, I suspect ArchLinux will have it pretty quick. Voice and Video chat is not yet in Windows.

Pidgin, got a major update last week that includes support for voice and video. Pidgin 2.6.1 also includes more than 200 bugfixes and patches, plus a few additional new features that may not be as sexy as voice and video but are still great to have.