A good cactus key is available here http://cactiguide.com/.

If I find any others in my travels I will post them here.

If anyone else knows any good keying sites please add them to this thread.

Thanks.

Here is one that I use, although it is only single pictures at a time, but good info on them at times.

Desert Tropicals

But is there a key where you put in info...

Like.. is it columar, does it have large spines, is it divided into 4 or more segments, does it have flat ribs.. so on and so on...

The first one makes an attempt at that... but it stops after the firts question then makes you search through pic after pic after pic in the hope you find your one.

Maybe you should make one slim6y

HAHAHA!!

I think I can succesfully identify one or two cacti (proudly).

So it will be a small key

Was only stirring you mate.

Another one I use as well as CactiGuide is Cactaceae.net.
There are a lot of names that are mispelt because its a mixture of peoples plants that have been uploaded but I have found it fairly accurate and there is'nt a bad gallery.

Another couple

http://www.cactuspro.com/photos__alpha__A1.html

http://www.astrokaktus.com/CactusBase/Vstop/a_cactusbase-en.html

These might be useful;
http://www.cactus-art.biz/index.htm
http://english.planetacactus.com/cactus_cards/fichas.html
http://www.bihrmann.com/caudiciforms/

I find cactus-art.biz very good, although it dosn't have everything and often focuses strongly on the mostrose/crested varieties of more common cacti, sometimes excluding the standard form altogether.