Anyone can sign up and use the Google App Engine. It works nicely on Linux.
You have to have Python 2.5, and you download their Software Development Kit. The applications that you build are in Python. After testing the application locally, it can be uploaded to Google. Of course, you can also point your own domain name to where the application lives as well, so no one will know that you're using Google.
Yeah, they do limit the amount of content to 500MB, which is enough for development. They're talking about increasing the content for a fee. So, you have plenty of space for development; but, when you land a contract, for your work, you'll have the option to increase the amount of space and pass the expense along to the client.