+1 for Sipdroid, I am currently spending a few months overseas and Sipdroid + sipgate.com work flawlessly for me every time. Sipgate actually gives you a phone number and unlimited incoming calls for free, and there are ways to integrate Goolge Voice to it (and they are having a promotion, you get an hour of outgoing air time free), between that and Skype, it basically covers all my long distance calls.
Alternatively, check with your cellphone carrier to see if you can get a femtocell. If you are with them for a few years, they will usually waive the device and monthly fee (especially if you call retention and threaten to leave). Then you will be able to use your regular cellphone.