Ok, here's Amazon's Free tier info.
http://aws.amazon.com/free/
You get 750 hours/month for CPU time. I think if you've got only one instance running, that should be all you need to get through. 613 megs of RAM. 5 gigs of storage and 15 gigs of bandwidth. All free for the first year.
After that, it goes to $0.02/hour for the time the instance is running (about $14/month) unless you prepay for the year and then it's only $54/year. This is all for their "micro" EC2 service.