I'm looking for a, preferably free, SIP client for OS X, one that will allow multiple SIP accounts and preferably supporting g729 audio codec.
Blink is a decent free VOIP client for Mac OS, though I'm not sure about g.729 support. For that you may need to look at something like Mirial. They have a Mac version but it doesn't have the Mac feel, and I think its expensive. Bria might be another commercial option as well.