Free (as in "beer") SIP clients are very scarse on the Mac OSX platform. Most clients are too far outdated and only work on old versions and on PowerPC. Other, more recent clients start up but they lack proper quality and functionality to be even remotely usable.
Luckily, there is a client available. Originally designed as meeting software using the H.323 protocol, it also implements SIP and can be used with any SIP provider, including Asterisk. The client is called XMeeting, and while it's no longer actively maintained, it still runs on Mac OSX up to 10.7 (Lion).
I've tested it out at work on our Asterisk server and it works flawlessly, and provides good sound quality, even on slower DSL lines. It's just what one would need to call via SIP.