Can't believe I haven't mentioned it yet... (and nobody else did)
Google Talk (downloadable windows client)
It's a Jabber IM client (can only use Google/Gmail Jabber account though, but of course you can communicate via text with users of other Jabber servers) that's, for me, as close to perfect IM as it gets.
Great VoIP - compared to Skype, it's equally fine on good connections, but much better on crappy ones. Small, simple, non-bloated, to the point, easy to install/configure/use - perfect when you want to talk via VoIP with somebody not computer savvy.
Unfortunatelly it doesn't have video yet, and its VoIP capability, based on open sourced Jingle library, is just starting to be implemented properly in other Jabber clients, so for now you're basically limited to original Gtalk client/Windows/Gmail Jabber account.