2D Software - Polycount Wiki

Description of what they are used for here

Package	Price	Website
GIMP	Free	http://www.gimp.org/
ArtRage	Free	http://www.ambientdesign.com/artrage.html
Inkscape	Free	http://inkscape.org/
Pixia	Free	http://park18.wakwak.com/~pixia/
Paint.NET	Free	http://www.eecs.wsu.edu/paint.net/
DogWaffle	1.2 - Free	http://www.thebest3d.com/dogwaffle/
Pixel32	$32	http://www.kanzelsberger.com/pixel/?page_id=12
OpenCanvas	~$42 (5,000Yen)	http://www.portalgraphics.net/en/
DogWaffle	Pro v3 - $97	http://www.thebest3d.com/dogwaffle/
PaintShopPro	$99	http://www.corel.com/servlet/..586/
Painter	$189	http://apps.corel.com/painterix/home/index.html
Photoshop	$649	http://www.adobe.com/products/photoshop/main.html

Photoshop is the most widely used.