I have an Android phone through Virgin Mobile with no contract (because I hate them, even though I keep my phones forever). They have several plans that range from $35-$55 a month. $35 gets you 300 minutes per month, with unlimited data and messaging; $45 is 1200 minutes and the same data and messaging; $55 is unlimited everything. It has 3G wireless, and of course WiFi and Bluetooth capability. It's an LG Optimus Elite (I think). The phone was around $150 and they gave me a $35 credit toward the first month's service.
"Maybe Lucy's right. Of all the Charlie Browns in the world, you're the Charlie Browniest."--Linus, A Charlie Brown Christmas
My fotki: http://public.fotki.com/nynaeve77/
I am not 100% sure how it works, but Straight Talk offers a prepaid plan for iPhones. Your phone had to have been through a certain carrier, and the initial setup may be a bit of a pain, but it could be worth it if you don't want to have a contract.
I just got a prepaid T-Mobile phone about a month ago, and I like it so far. I don't get reception at a state park out in the boonies that I visit sometimes, but I didn't with Sprint either.
The service. When my son went a couple of months w/o using the phone (it was for emergencies only), T-Mobile deactivated the number and reassigned it. I wasn't informed of this practice and I didn't find out until I put money on it and it failed to make calls. Getting my money back was a nightmare and some of the reps were tough to understand, so there were communication issues. I finally had to do a chargeback on my credit card. Since the phone wasn't jailbroken, I had no choice but to continue business with T-Mobile and pay a reactivation fee. I've had other issues too, but they're too convoluted to go into here.