Normally $15. (Which is still a good deal!)
http://store.steampowered.com/app/431730/Aseprite/
By David Capello. (Who also participated in Speedhacks as recently as 2007.)
It's a nifty sprite art tool written with Allegro. I just got around to buying it with the Xmas sale. I played with it for a few minutes and I already like the feel of it. The buttons you need, with simple hotkeys, and everything else is out of the way. Just picking up pixels, and dropping them. I made a cute little wizard sprite by just starting with a blob and then thinking "what does this kind of look like?" and then just trimming it and shading it and boom. I'll post it when I get back up in the morning on my desktop PC.
It supports animations, onions, and a bunch of other stuff that programmer-me barely understands. But I definitely feel at home with it compared to 90's/DOS graphics tools I used to use for modding games / making my own sprites.
There's a free/trial version on his website, the second link if you want to try-before-you-buy.
Supports Windows, Mac OS X, and Linux!
Here's the version roadmap (currently 1.2, with crossed out pieces being done)
https://www.aseprite.org/roadmap/
And I just realized he's open-sourced it on github with issue tracking/bug submissions:
https://github.com/aseprite/aseprite
Pixel art? Allegro? Open source? Linux and Mac OS X ports?
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Down boy. 
This is the first I've heard of a new version of ASE. Looks like it came back from the dead. Might give it a trial run. Paint.NET does everything I need it to for now. If I was developing a game, I'd probably buy the full version. $15 is pretty cheap. Lifetime license? Free upgrades? Or BS?
.gif or a png sequence? Are there any .gif libraries for A5?
FWIW, if you're cheap you can just compile it from source for free (can't redistribute it though as per the license). It's a great tool, have been using it for the past few hacks and it really boosted the creation speed and quality of my graphics.
How does this compare to GIMP for making pixel art? I use GIMP almost exclusively, but must confess that it tends to get in the way more often than anything else.
Edit
I checked out the trial. It's pretty cool. I think I'll buy it from the site instead of Steam.
I used to use GIMP before I switched to ASESprite. The biggest advantages are that ASESprite is oriented around animation and tiling, so making either is particularly easy in it. It can export tilesheets etc which are a pain to do in GIMP. Other nice to have things are the ability to draw lines/circles/rectangles easily.
That said, ASESprite is really basic in many other ways, e.g. compared to Moai: https://www.allegro.cc/depot/Moai which has a lot of fancy effects. Moai is very complicated though, and Windows only (works fine in wine obviously).
I bought it on Steam, but can't say I've actually used it....yet. I got it shortly before I bought Pro Motion NG and FL Studio 12. Been playing with both of them just to learn their UI, but haven't taken time to think of a game that would give me a chance to use any of them. I only got ASESprite for the fact that it was made with Allegro, so it deserved support.
I use Allegro. Therefore I deserve support too. Donations may be made via PayPal. Thank you very much. 