I like
www.iconarchive.com
Many high quality png sets free, or free for non-commericial use.
Many sizes
Flags
http://www.iconarchive.com/show/all-...on-design.html
Files
http://www.iconarchive.com/show/mnem...echiceroo.html
Really, it's hard to choose
http://www.iconspedia.com is another good source
I use the icon sets for my CustomUI (Fluent) icons in the MS Ribbon