SQLite: An embeddable database
OK, this has been written about by others, but it's so cool, I had to mention it myself!
As the home page explains, "...SQLite is a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine.." It's ACID compliant, the data files are portable across different machine architectures, it can handle respectable amounts of data (up to 2 TB) and it comes with Python bindings. :-P
It's seems like a great way to store application data without having to force users to install MySQL/PostgreSQL or hacking up a custom (probably buggy) solution.
I can't wait to use it!