[SHARYNF]

the absolute simplest is a wall wart and a pot. but if you want to get a bit more sophisticated here is a replacement for the pot

http://www.edn.com/archives/1997/102397/22di_04.htm

Here is a dimmer from boats that use 12 volts

http://www.sailorsams.com/mall/dimmer_d1201.asp

here is another option
http://www.platinumlightinginc.com/DIMMER/dimmer.html

Sharyn