I don't use Ubuntu, but I've just been setting up SIP myself. My current favourite software is
Twinkle, and I'm about to try the latest version of
Ekiga (formerly known as Gnomemeeting).
I've been trying it with sipgate.co.uk. The only problem is, Twinkle doesn't (at the moment) support the method of sending DTMF tones that sipgate require, so it makes accessing some services difficult. Whether this is a problem depends on your SIP provider.