As Two2seven said Appshopper is great. You can set up patched apps that will give you a email alert when they come down in price (just gave me iAssociate today).
It also has the really handy 'Popular' list where you can see apps that have dropped in price filtered down to the ones that a reasonable people Re actually interested in.
(You can get the full list too - as loads of other apps will do too - but there are way too many of those)