The Open Game Art Bundle
The Open Game Art Bundle is a pack of game resources that was funded by the public, for the use of the general public. The above picture is supplied by the creators of the picture, to show that you are free to use the works included in it however you wish. Every work in the bundle is released to the public domain under the CC0 (Creative Commons 0) license. This means you are free to use the entire bundle in ANY kind of game, commercial or not. You can alter the bundle's contents. You can share it (Like I'm doing now!), you can sell a game that uses these resources an you are not required to pay any sort of fee. It's all free.The only thing you're not permitted to do with these resources is sell them as is. They are released to the public as free for anyone, not for scamming and deceiving others. Use it respectively. The creators of this content are doing you a favor, don't insult them for their kindness.
You can get the resources here!
Resource Examples (unfortunately, we can't use iframes to use Soundcloud's embedable player):The original soundtracks from Adventure Story,Bullet Heaven and Epic Battle Fantasy III.
The soundtracks and the full source code from Reimagine: The Game. (Download the pack for the source code of the game. Also note that it is a flash game)
Two video game soundtrack albums and a collection of music loops.
And there are still the graphics resources waiting on their webpage!
Crediting the Artists
Just to clarify, this pack of resources is released into the public domain. You are not required to credit anyone if you use them. You are allowed to credit the makers of the resources you use, but you are not required to do so.