I love Pyxeledit. It's not free, but it's only about $9. It's brilliant for mocking stuff up since you can create your own tiles n' plaster them about, then, if you edit one, they all update to fit the changes. the sprite animation and export options are brilliant too. It's in beta though, so there are a few tools n' features missing, (like flipping horizontally/vertically), but it's the only program I use for my pixelling.