In 2007? For me that was Pidgin, because before that (and still on my PC) I could only log into one IM account at a time.

In 2008, I'd cast my vote with Canola. I'm tickled that I can download podcasts right onto my IT with it instead of using my laptop and then syncing.

(But really, the real beauty is the IT itself. OMWeather, email, browser, IM, gnumeric, etc... I use them all every day on this thing. How did I live without IT?)