Windows only: Portable software usually has to suffer a few through a few compromises to be compact and flash-drive friendly. If they compromised with Fotografix, you certainly won't notice.
The interface will be familiar to users of bigger graphics packages like Adobe Photoshop and GIMP. If you've never used either one, it won't take much to pick it up. Fotografix is astoundingly tiny—a mere 680k when unpacked—for offering features usually only found in bulkier and more advanced editors like image layer, custom brushes and advanced colour and image correction tools.
The advanced features of Fotografix cover enough ground that for anything short of having to deep massage an image in Photoshop, you'll likely not have to fire it up any time soon. Fotografix makes an excellent and ultra-lightweight addition to your portable software package. If you have your own portable software that accomplishes a ton with a tiny footprint, let's hear about it in the comments below. Fotografix is freeware, Windows only.