you can try heywire. Works on wifi, but it does give you a number to txt with.
If ANYBODY here has a better one, please post it. I need one also. Giving my kids my Droid 2 to use as an ipod and currently will be using heywire so they can at least txt. But I would like to find one that will also allow them to make phone calls with WIFI ONLY (gosms for example will not work unless theres a cellular service).