Google Talk
Google released their IM/VOIP client yesterday.

What I like:

• Seamless integration with my contacts in Gmail — no need to recreate a contact list. For instance, my realtor has a Gmail address, he has Google Talk, and he is an email contact of mine. He appeared in my IM contacts with no effort.
• Voice over IP client. If my sister gets a Gmail address and has Google Talk, we can talk online for free. Hint, hint.
• Simple interface, light install — multiple chat windows dock with each other

What I don't like:

• Some parts don't seem fully functional; the email button doesn't work for me, for instance.
• Timestamping functionality is limited

I read that Google has plans to allow for shared web searching while chatting. That would alleviate the classic “hold on while I Google the link for you and paste it into the IM window” problem.

Also, you don't need an invite for a Gmail account any longer. You can sign yourself up at gmail.com