odds that another flush draw is out?

[tyler_cracker]

I can't figure out the keywords i need to use to look this up on google or here, so i'm just going to ask.

If you have a flush draw (2 in your hand and 2 on board), what are the odds that an opponent is drawing to that same flush? Surely there is a page that lists the various possible permutations of this question as well (e.g. odds of two made flushes when you have 2 in your hand and there are 3 on the board).