When starting new with my Rails environment, I decided to try to use SQLite as my database rather than the more typical mysql or postgres. There’s no need for the latter as I just need a single user, embedded database to do dev and testing on. Since Locomotive ships with sqlite right in it, it was easy to set up and try out. Google Gears and the upcoming Firefox 3 both use SQLite as their embedded database engine so this is not a tiny open source project.
Unfortunately my experience has been less than stellar. It appears to work flawlessly except for a single bug which is a complete showstopper for Rails applications. The SQLite development team seem to be in no particular hurry to fix this bug so I’m afraid I (and many other Rails developers) will have to avoid using it. 