There are a ton of PS2 peripherals out there.  The original Sony PlayStation DualShock gamepads are some of the best controllers ever made.  Now you can use it with the Wii as a classic controller.  Not to mention all those awesome arcades sticks that are now available to you.  The PS2 has a huge collection of arcade sticks of every style you can think of.

I play Tatsunoko vs Capcom and now I can use my homemade PS2 sticks with the wii.  An added bonus is now these peripherals are wireless!  I got these adapters from DealExtreme for about $8.

They work by plugging into the wiimote through the nunchuk port and then you hook up your PS2 device.  That’s it.  You can now use your PS DualShock to play wiiware games.  Along with gamepads and arcade sticks, you now can use PS2 guitars with the Wii.