Danc's Miraculously Flexible Game Prototyping Tiles

In fact, he's posted numerous collections of free tilesets and sprites, in a number of genres. For example, graphics from an unfinished RTS game:

    http://lostgarden.com/2005/03/game-post-mortem-hard-vacuum.html

A 2D top-down medieval RPG:

    http://lostgarden.com/2006/07/more-free-game-graphics.html

Graphics from Tyrian, a vertical shooter he worked on:

    http://lostgarden.com/2007/04/free-game-graphics-tyrian-ships-and.html

The full list of free-game-graphics posts:

    http://lostgarden.com/labels/free%20game%20graphics.html

The update also has an explicit license, in case you want to distribute any game you make.