More options is great. I also used to use pbexes and google voice, but that is down to at-least May 2011 for free users. Auto account generation has also been taken down.
Now I use
Sipgate and Google Voice. You can signup for a FREE Sipgate One account. Incoming is free, and you use Google Voice call-back for free outgoing. I have found the latest version of Sipdroid (2.2) to be bugged with sipgate. It often tells you your SIP is registered but it really is not (checked on sipgate's site). I do not have this problem with Sipdroid 2.01.
There is also
Whistle for USA residents. It has a native Apple app but you can also use SIP (for us Android folks). But the incoming calls can be hit or miss for some reason. It plays a ad on some of your outgoing calls before the connection. But it's free. I am assuming it can also be setup with google voice if you prefer.