[Gabethebabe]

if you have Excel, you can make a spreadsheet with which you can make lotsa calculations.
Construct it like this:
Cell H2: No of cards in the deck
Cell H3: No of outs remaining in the deck
Cell H4: No of card that remain to draw
Cell H5: No of outs you want to draw
Cell H6: Chance that you draw these outs.

Paste into cell H6 the following formula:

=FACT(H2-H4)*FACT(H3)*FACT(H2-H3)*FACT(H4)/FACT(H2)/FACT(H3-H5)/FACT(H2-H3-H4+H5)/FACT(H5)/FACT(H4-H5)

Nou you can make calculations.
For example. I have KK and my opponent has a weak ace. What is the chance that he flops one ace?

Cell H2: No of cards in the deck = 48 (52-4 hole cards)
Cell H3: No of outs remaining in the deck = 3 (3 aces)
Cell H4: No of card that remain to draw = 3 (the flop)
Cell H5: No of outs you want to draw = 1

And out flops the answer.