Basebot is a set of laser cut templates, hardware recommendations and guides and software for a cheap wheeled robotics platform.
Get the SVG or DXF files for the acrylic parts from this repository, and see the wiki for the bill of materials and assembly instructions - these don't currently include control or power circuitry, but you'll end up with a solid chassis including motors ready for your controller of choice...
The latest version (v16) looks like this (lacking any form of control circuitry!)
This has evolved quite a bit from an earlier version (v4 or so)
Developed at Makespace in Cambridge, UK (http://www.makespace.org)