Hello GatKaptain. We actually have a thread showing Pixel Art tools as well as other useful bits here:
http://wayofthepixel.net/index.php?topic=3467.0You could also glean some information from the thread PsylentKnight posted I'm sure.
The most popular ones seem to be photoshop, paint, GraphicsGale, ProMotion. Tiled is a good program specifically for backgrounds and mock ups.
Please be sure to check out all the resources the forum has already provided before making a thread!