Those interested in Pixel pushing and the old school, there's a lot to chose from, especially free programs.
It's something well worth playing around with, especially handy if you are doing tile maps for hand held devices.
Tile Studio - This is a nice open source freebie
mtPaint - This is also open source and works well, just doesn't appear quite as polished.
fishEd - This is great and recently released as a freebie. Lots of resources here on the site too.
Graphics Gale - A nice little animation pixel program. Not free though, but not expensive either.