FlashFlashRevolution.com, Multiplayer Online DDR Simulator
Stepmania, Free DDR Program, Same engine as In The Groove, songs must be downloaded
DDRFreak.com, songlists, FAQ's, and general tips for DDR
ITGFreak.com, songlists, FAQ's, and General tips for ITG
RedOctane.com, maker of high-end gaming peripherals, good softpads (V3 has USB connectivity)
Cobalt Flux, High Quality Dance Pads, I personally use one