Mainly ICQ, but I also have MSN and XMPP at hand (though the latter appears to be unavailable from campus). I have Skype as well, but have not installed the program on Win7 yet. Also, I hardly ever run it on my laptop.
The client I use for all of these is Trillian Astra, but I never used Trillian's new Astra network for actual communication, only connecting for contact list synchronization and other backend services.