(obdisclosure: I'm a Google employee, but the usual these are my words not the company's blah
Slashdot had a post of someone porting appengine to Amazon's EC2 cloud. I'd be really
surprised if some "GNU AppEngine" for standalone hosting on free platforms didn't show up.
Something I do like about Google is that they've (we've?) been pretty good about publishing
the APIs that are used and not minding when other people implement that API themselves.