Yusuke Kamiyamane is sharing two huge sets of free PNG icons for websites & applications. These two sets contain 3046 icons in total:

• Fugue Icons
  Set of 2,646 icons in PNG format in 16×16 and 24×24 & available with PSD source files.
• Diagona Icons
  Set of 400 icons in PNG format in 10×10 and 16×16

If someone plans to use them, only a back-link is required to use for free else you need to purchase these icons for a small price.