That I am aware of there are 3 different versions of Fluxx Online at the moment.
The one that I designed is at www.ccgworkshop.com. It is Windows based and has many great CCGs on there as well.
Volity Fluxx is another new program that has Fluxx and is available on most platforms at www.Volity.net
Steve Mattison is also working on a project called Fluxx: FPR @ http://www.freewebs.com/zombieflesheater/fluxx.html
Being the designer of the CCGworkshop version, I like that the best. It give the ability to make any deck you want out of the cards available from any set. The only downside that I constantly here is that it is for Windows only. Try them all out and see which you like the best. Or if you really want a challenge, design your own.