[JoshuaD]

Just multiply by 2 for each card left to come. If you've got alot of outs(9 or more), it's a little less, if you've got only a few outs (3 or less), it's a little more.

So 8 outs on the flop comes 2 * 2 * 8 % of the time by the river, or 32%.

It's just an estimation, but it's a pretty good one.