The best replacement for ResEdit I've used is Resorceror, but it's awfully expensive.
Another few are ResFool, ResFork and Rezilla. I can't remember if/when I tried these three, so I can't comment on their quality.
HexEdit can also be useful for editing resource forks, but it's strictly a hex editor and not really geared towards managing resources like ResEdit. It's definitely worth a download anyway, though.
If you want to mess around with the interface of applications and change it around, i would recommend Apple's Interface Builder, much better than resedit in hacking cocoa applications.