If you have d2a, any chip will work.
If you have d2b, you will have to solder to cut legs, requring a dremel. Search for an install guide, you basically have to trace where the points would have been. See bottom of post. Thanks ekinplektrum for those links.
If you have d2c, you need to use this chip: http://www.d2ckey.com/ , which is like 30+ wires to solder.
Or this one, argon, which is only 13 wires http://www.postish.com/Browse/ViewLi...spx?itemid=835 (probably available elsewhere - just google Argon 13 Wii)
Or D2Pro, made by the Wiikey team - still 13 wires.
Or Wasabi - Apparently the best now.
Check Modchipcentral to find the latest chips, as new D2C ones keep being released.