Smashing Magazine occasionally posts really nice icon sets. (Just do a search for "Icons")

Iconfinder
The only place I know where to find icons.  Never really needed anything else.  It's the 'Google' of Icon search! ;)

Just like Cy Klassen above me, I use Smashing Magazine very often, to find the right icon-set for my projects

Another good tip I can give you is to search for "icon set" on delicious -  there you will find many links to blog posts with every kind of icon sets!

Iconpot is always worth checking too.

Iconfinder works for me ;)

We made them ourself.

I make them myself, that's the way to truly win-out. Otherwise, if you're doing something less important you could try famfamfam