The Windows Mobile Operating System comes with MSN Messenger, so if that's all you're looking for, you don't need to install anything else. If you're just looking for AIM, get it free from
here.
Agile Messenger and
imov messenger are also popular choices if you want to connect to multiple IMs simultaneously (e.g. MSN, ICQ, AIM, Yahoo).
IM+ also does the same thing but is payware.
For a Skype alternative, I'd recommend
Microsoft Portrait, which is also free. I've used it very successfully with voice and video. :approve: