There was a push a while ago to get Drupal to use the tango icons, which was then pushed back, since Tango’s license is CC and not GPL. I don’t fully get the distinction, but here are the GPL icon sets I have found:

• Lullabot Icon Set #1
• Morten’s Iconify Module
• Mark James’ Mini famfamfam set

Other nice free (but not GPL) sets

• Drunkey Love
• Mark James’ Silk