http://www.geeks.com/details.asp?invtid=PCJOYPAD-BLU-N&cat=JOY&cpc=GAM$2.99 if you buy it during the green light special. 4 way gamepad plus 10 independent (and recognized by winXP) buttons makes it a cheap 14-input USB gamepad hack candidate. Looks to be at least as easy as a keyboard hack, although I haven't opened it up.
Or get two like I did for laptop travel mame satisfaction -or- to plug in for players 5 and 6 on Xmen when you normally have a quad control panel.
Recognized and all 10 buttons are independent, including SELECT and START (TURBO, SLOW, and CLEAR not recognized) under WinXP. I will report on advj compatibility under linux soon.