CNC Router parts for sure. My machine is a clone of those kits that I started building before CNCRP started selling kits.
Personally one of the largest benefits to these style of machines is how modular they are. If you started with a 4x4 to save $$ and later decided to expand to a 4x8 or whatever you can fit in your shop, the investment is pretty minimal.
Also the fact you can disassemble the machine and move it is a good bonus. Not to mention you don't need to glue or weld as with other machines. Aluminum extrusions are the bee's knees. =D
My 8x4' 8020inc | CNCRouterParts R&P build log: http://www.cnczone.com/forums/open_source_cnc_machine_designs/126280-4x8_8020_r_p_cncrp.html