Every major instant message protocol, comfortably crowded under one (free!) roof. The addition of push notifications notched this one up from great to, uh, greaterer.
Obviously Skype is still your safest bet for making Skype calls, but Fring can do it passably well, too. Most people were excited for push notifications precisely because of how they could used for instant messaging, and Fring more or less fulfills this vision for free.
From the App Developers: fring is a mobile internet community and communication service that allows friends to connect, share experiences and enhance their communities (Skype®, MSN Messenger®, Google Talk™, ICQ, SIP, Twitter, Yahoo!™ , AIM®) together, using your iPhone's & ipod internet capability.