WAIEMU has several files worth checking out made by Ken and his friends.
The successor of TournaMix and DDROSC. BLueSS runs the best contests around.
Zenius -I- vanisher.com
The place to go to for official and original stepfile downloads and Bemani news (mostly DDR).
Another place to get and upload files to (although most of them are pretty bad).
You need this to play stepfiles!