A simple tileset published for free by IndieGameDesigner. It contains Ground, Stone, Ice, Cracks, Water and Clouds.
A huge set of more than 800 seamless tileable textures that can be used for games, created by Patrick Hoesly. All the textures are hosted on flickr, well organized and comes in resolutions of 500×500 or 600×600 pixels. Check author blog for more details.
The tool is no longer available, but you can download a few pregenerated sprites.
The Explosion Generator from phaedy is another tool that can be used online. It accepts a few parameters that can give you explosions that quite different then exports it as a transparent png.
Walking/Jumping Robot Isometric Sprite
Those files were removed due to copyright issues. I appears they are NOT FREE TO USE. Original link to the resources is removed because is misleading.