A simple tileset published for free by IndieGameDesigner. It contains Ground, Stone, Ice, Cracks, Water and Clouds.
A tileset for platform games.
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.
Another free tileset aimed for RPG games. It contains 7 characters and 4 sets of tiles: farm, interior/exterior castle and town.
A nice explosion online generator inspired by metalslug. It generates an explosion sprite based on and random number that can be saved as a transparent png file. Beside the random number there is no other parameter to configure.
A tool that generate FX sounds in the retro style. You can generate random sounds or adjust up to 22 parameters to find a sound effect you like.
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.
A simple silhouette animated sprite or a walking/jumping man. It contains the bitmap sprite and a flash version.
A basic ground tileset containing 3 tiles of 50×50 pixels. You can use in platform games, side scrollers or any other type of game requiring a simple ground view.
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.