Whistle for iPad is good and free. It just has a walgreens advertisement in the middle of the screen.

Acrobits Softphone is quite nice too, for $6.99 I think. It offers tons of official support for lots of SIP providers, and push notifications for incoming calls. Though I've found push to be worthless for actually receiving a call in time.

Then there's Siphon in Cydia but I haven't really used that.