With my Galaxy Nexus and Rebtel credits, I would call Chuck Norris to let him know there is a new badass in town...and it's name is THE Galaxy Nexus...then I would change my number and go into hiding from Chuck Norris.
I would call my Boyfriend that is living in the UK right now. We call each other on Rebel and it works great. Free Rebtel to Rebtel calling made it totally worth having.
My entire family lives on another continent, haven't seen them in 12 years now, so I'd definitely call them.
Heck, I'd probably use up the 100 credits in one week and have to purchase more
I think I would be too distracted by the awesomeness of the phone to be able to put the credits to use.... but when I finally snapped out of my fascinated state I would probably call a friend who is generally hard to reach because she is an airline stewardess, and out of the country 90% of the time.