open-source-rover - A build-it-yourself, 6-wheel rover based on the rovers on Mars!
RepRap is an open-source (hardware, software, and documentation) desktop fabrication system which lets you construct arbitrary objects (or parts for assembling more complex systems). It's unique in that you can construct one yourself for about $400us, and it is fully capable of making duplicates of itself.
An open source DIY robotics/rapid prototyping kit much like an Erector set. Lots of standardized pre-milled bars, rods, connectors, and components. Build your own plotter, CNC machine, RepRap, scanner table, or what have you. Capable of three axes of linear motion. Comes with all the electronics you need, including an Arduino, stepper motors, and a motor driver board.
A browseable online database of servomotors and their operational specs.
lcamfuf wrote a book on some classic DIY/maker techniques for building stuff, such as CNC machining, mold making, and resin casting. Cost is a consideration in everything he wrote about so if you're not rich or well connected but want to get into building things, give this a read. You can read it chapter by chapter or all in one go.