For the longest time I would point to heroku, but now that they're obviously pinching pennies I've decided to take a second look at the landscape.
I'm having a hard time finding a good offering anywhere else. My main criteria is to be able to simultaneously host many small projects, and easily scale if/when one succeeds.
AppFog was the closest one I've found that makes sense, but their model of having all your apps share resources makes pricing confusing.
EngineYard looks OK as well, but their pricing is also a bit weird and also seems too complex.
I like process based pricing, because its easy for me to see how much something is going to cost me.
find some more alternatives here: http://blog.fortrabbit.com/comparing-cloud-hosting-platforms...
plug: i am co-founder of fortrabbit (PaaS).