Mobile Freedom OverviewHere are some options for setting up a data-only phone to make calls over wifi with no contracts and no monthly plan. Certainly there are other approaches, these are just the ones I've tried myself.
option 1a: Get a parked SIP number (SipSorcery, IPKall) that you can use for Google Voice call forwarding and piggyback on free wifi networks for data (office, library, McDonald's, etc). Combined with a month-to-month wifi hotspot (DataJack or Virgin Mobile), this is my favorite option.
options 1b: Same as above but use a SIP client like SIPdroid or Softphone instead of Google Voice. This option won't give you SMS or voicemail transcription.
option 2: You could skip SIP altogether and use Skype with a online number set up. Skype is decent over wifi but atrocious on 3G hotspots. You also can't get incoming SMS messages. Not my favorite option.