A command-line based data recovery utility for Linux's EXT2/3 filesystems. Can recover all of a user's deleted files, deleted files based on type (.mp3, .jpg, etc), can even recover dropped MySQL database tables.
An excellent blog post on what to do if your MySQL database server isn't working right because an InnoDB table has been corrupted. I've used this process myself and it works like a charm. Knowledge of MySQL in particular and RDBMSes in general is required.
This site collects CNC and laser cutter meshes that can be used to manufacture open source office and home furniture.
An opensource tool for the visualization of extremely large datasets, like twitter maps or email databases.