Looking for a special icon?

You could try Icon Finder.
I tried it just once and, although it found two of the icons I was looking for, I know there are a lot more out there that it didn't seem to find.
Perhaps a more complete survey of what's available is more appropriate. In that case look here.

Edit (22/05/08): Here are some more good quality free icons.

Edit: (23-05-08); And yet another one. This one, IconLook does look different (different as in Better).

Edit: (30-05-08) Use this tool to make your own quality icons.

Edit: (12-06-08) Some more but these are of very hgh quality.

Edit (23-06-08); Some more here.

Edit: (24-06-08) And here are some more.

Edit (3-08-08) Some very nice Linux icons

Edit (10-08-08) Here are some more. These are of high quality and some are large.

Editr (7-09-08) And here's some more of medium-high quality.