HBase

HBase, yet another alternate distributed column-oriented database technology, inspired by Google's BigTable. It is hosted within the Apache Hadoop project (MapReduce implementation).

InfoQ article about HBase.

Interesting excerpt about Google's App Engine:

However, as a lot of people have noted since the announcement of App Engine, there's a big difference between owning your infrastructure and renting it. It's probably a very good thing for you when you are small, but as soon as you reach a surprisingly low threshold, you're better off hosting it yourself.