PortableApps.com is popular for their wide range of portable applications that they have available, such as Portable Open Office and Portable FileZilla. Now they have released a portable version of GIMP. If you aren’t familiar with GIMP then this is the easiest way to test it. It has many similar attributes to Adobe Photoshop although it does lack the simplistic interface that I am accustomed to in Photoshop. If you have never used Photoshop before then I am sure you will love this free application for editing your photos, however, existing Photoshop users may find it difficult to adjust to the differences.
I am currently using Portable GIMP because it is a nice replacement for Paint in Windows. That way I can carry it with me on my USB drive and always have a decent photo editing application.