SQLite

Widely recognized as the most lightweight and speedy of all open source database engines, SQLite is rapidly gaining popularity among web developers. Without sacrificing too much in the way of functionality, the SQLite engine is able to handle data storage efficiently and dependably.

Instead of running in a process like most database engines, SQLite embeds itself as a library during a script's execution. And since SQLite is run under a user's process, databases are stored directly under a user's directory in the form of single files. Due to this unique design, SQLite allows for a finer degree of control and portability than other solutions.