Use your Playstation / PSone / Playstation2 controller on your Xbox, just plug it in & you're ready to go.
Compatible with every Xbox console.

Compatible with Dual-Shock & Dual-Shock2, also works with fine with non-Sony brand controllers I've used.
Even works with steering wheels.
Supports all of the analog(pressure-sensitive) buttons of DualShock2 or equivalent controllers, including triggers, to have same analog effect with Xbox.
Also supports vibration.

Compatible with all nearly all Playstation controllers - except unusual items such as possibly some wireless controllers(works fine with ones I sell at Feb 2010), some arcade sticks, & some dance mats(works fine with ones I sell at Feb 2010).
Probably won't work with any Playstation light guns.
Unlikely to work with any non-analog(eg: old controllers without thumbsticks) controllers, as xbox games are designed for controllers with analog controls.

Button mapping is almost identical to using a normal Xbox controller:
directional controls(up,down..) = the same,
L & R analog sticks = the same,
L3 & R3(pushing down on L/R stick to make them click) = the same,
start = start
back = select,
L and R trigger = L2 and R2,
black and white = L1 and R1,
Action buttons - A,B,X,Y = Square,Triangle,X,O(same layout)

Will not work with Xbox360