An interesting review of 39 free softphones by VentureVoIP. The softphones mentioned are all free downloads that do not require any form of registration.
The most well known on the list has to be Counterpath‘s X-lite. X-Lite 3.0 lets you make audio calls to any softphone, mobile or landline number, and make video calls and send IMs to any softphone which supports these functions.
Firefly is also apparantly popular amongst IAX Asterisk users and recently also offers support for Mac, where previously it only supported Windows.
PhonerLite is notable for being one of the few with support for the G.722 wideband codec.