Here’s six good resources I’ve discovered:

1. IconFinder
2. Social Media Icons in 3 Sizes for Download (and version 2)
3. The Best Social Media Icons All in One Place
4. 80 Refined Social Networking Icons for Free
5. 30 Amazingly Creative Social Bookmark Icons Sets
6. The Best Original Freebies

Got any other recommendations?

Update:

• Default Icon is another great repository for basic icons that can be used on multiple platforms.
• Iconlet.
• Icon Scan.
• IconSeeker.
• Icons-Search.
• Mr Icons.
• Very Icon.
• WooCons – 170 free icons from WooThemes.
• WPZOOM.