SQLite is an amazingly light with very rich functionality relational database. I have been using it for quite some time now, with PHP and with Java.

Recently, I saw that Firefox3 uses it very extensively for its own management of Bookmarks and other things. Firefox3 has multiple instances of SQLite.

The best thing that I like about SQLite is that the entire database is stored in a single file on the file-system.

SQLite is a single file database with APIs in C, Java, PHP, and Python.

I use it for my desktop applications, when they are small. SQLite supports SQL commands, and hence can be migrated to a full featured database like MySQL, if the database grows.

SQLite is perfect for those small database chores where only a single (or small number of) processes will be updating a database on your computer. While its architecture is quite nice and works quite well (even multiple programs can access the file and semi-simultaneous commit changes), it is not in the same class as those that are currently being offered up as "alternatives".

It is embedded in many applications, both open and closed source. If you dig around deep enough, you'll see it behind Apple's Core Data service as well as the iPhone. That a lone means that it is probably deployed on more systems than most other database systems :-).