I use OctroTalk or Palringo... both are free (although I think octro will eventually go pay) Octrotalk is at
Octro and palringo is at
Palringo Vocal IM. Be aware that on my wing, once I installed palringo onto the storage card, it had issues as it wanted to start at boot, so I recommend either installing it into main memory if this is an issue for you and you want it to start automagically or install it to a storage card and removing the startup entry so that it doesn't start at boot. Both support all the main services - MSN/Yahoo/Jabber(googletalk)/ICQ. Neither is a huge draw on the battery when connected all day although I'm not sure how much data is transmitted (if you don't have an unlimited plan) so that may be a consideration for you.