I've been using SQlite more and more when building short or temporary projects. It's great, light, and I don't need a database server running, which is really nice. (It should be noted that I know almost nothing about databases, so I may be using SQlite to my detriment.)

I just found a pretty nice Firefox plugin: SQlite Manager. It provides a gui interface to your SQlite databases so that you don't have to remember the syntax to deal with them from the command line.