There are several options now. The most prominent being fring and Nimbuzz.
I'm using Google voice + Gizmo5, and in my experience, Nimbuzz has been hands-down the best SIP app. It has better call quality, the most stable network, and a crisp UI. fring is pretty good too, but has had some on/off network problems. Their voice quality seems inferior even when the network is smooth.
One caveat: if you aren't on either an iPhone 4 or an iPhone 3GS, I can't recommend Nimbuzz, as they limit push notifications (and therefore incoming calls when backgrounded) after 4 hours if you're on an iPhone 3G or iPod touch. Not sure what the reasoning is there . . .