For instant messaging I use the GTalk app that comes as part of Gmail and eBuddy Messenger from the Android Market.
eBuddy Messenger is useful since it can be to simultaneously chat with MSN (Windows Live Messenger or WLM), Facebook, Yahoo!, MySpace, Gtalk, ICQ, AIM and Hyves. No file sharing or video chat, but it keeps my instant messaging in one place.
eBuddy Messenger can connect to GTalk, but I use the separate GTalk app since I use that for office communications so it is on 24/7. When I do not want to be bothered, I turn off eBuddy Messenger.