Chat and VoIP program Nimbuzz connects to Skype, standard SIP VoIP, GoogleTalk, Jabber, MSN (Windows Live), Yahoo, AIM, ICQ, Twitter, Gadu-Gadu, and chat of social networking sites Facebook, MySpace, orkut, and Hyves.
Update: fring is no longer a multi-network messenger. They kicked 'em all out, fring now only works on it's own fring network. My advice: don't use fring.
The WeFi plugin for fring is a nice touch for those with Feature Pack 2 phones (Nokia N79, for example), because there's still no FP2 version of the original WeFi application.
Nimbuzz has a somewhat better user interface than fring, and the sound quality is better too. But fring has more features and it is sometimes able to connect through firewalls that Nimbuzz cannot pass. If you can't choose, just keep both programs on your phone.
Nimbuzz and fring updated their programs recently, but they didn't add new features, and the user interface didn't change either. The changes (bug fixes, stability improvements) are all behind the scenes.