I use Adobe Photoshop CSS2 & CS4.
Dynamic Photo-HDR
ImageMagick
DigiTools for Photoshop
FastPicture Viewer 64
LightBox 2.0
NeatImage
I also use GIMP and/or Paint.Net on occasion. But a majority of the work I do ends in print, so I really have no choice other then Photoshop since it is the only software with soft proofing and totally profile aware.
If not going for print, GIMP or Paint.Net offer pretty much everything PS does, but both cost nothing.
The newest program I have been toying with is LightBox. v2.0 is free now with v3.0 being sold for a discount currently at $39.95(saves you $10). So far it is a nice software. Very much reminds me of Adobe LightRoom! I am thinking of purchasing v3.0 as it is fully profile aware which means it will be suitable for editing for print and it edits natively in 48bit!! I don't always need all the benefits of PS and LB is quicker to load and faster to process.
Sagelight 48-bit Image Editor 3.0