I think that 750 limit resets every month for a year though. Thus you could get by on one of their micro instances for a year for free. Not sure if GAE or Heroku has a similar limit.
Heroku limits you to one dyno, which is equivalent to one server instance (mostly to one server process). So if your app is serving requests from Ruby, then on the Bamboo stack at least it can only serve one request at a time. For static content this doesn't bother me though, as on the Bamboo stack you also have Varnish, which has great async behavior a really good performance for serving static assets.
GAE has daily limits. For instance you have 28 frontend instance hours, which get depleted quickly when using multiple instances. You also have a 1 GB bandwidth limit, which apparently is really insufficient for serving images bigger than small logos (looking at their dashboard now). And the list goes on. It's OK for low to medium traffic, if you're not doing lots of background processing and if you're not serving big files.
AWS's free tier allows for renting a micro-instance for a whole year. However the micro instances, as I discovered, have really poor performance characteristics. They work in bursts of available CPU. So one minute they may have better performance characteristics than the next instance type and the next minute a micro instance can get completely frozen with pending activity.
I don't like AWS micro instances. You're much better off paying for a small Linode instance, something like $20 / month.
