Up until about a year ago, I've been using the bog standard Windows Live Messenger. I was fed up with all the bullshit features and spangly bits, so I downloaded A-Patch to get rid of a lot of the fat, but it still ran like shit and was a killer on system resources. Afterwards, I downloaded Pidgin Messenger, which I also use when I'm on my Linux machine. I find it to be extremely light, has all the options and settings you need, and has some kick ass plugins! If I was to recommend anyone an Instant Messaging client, I'd pick Pidgin over the lot. You can even use it with TOR, as well as the OTR Plugin which is badass.
So, what do you use, and why?