Dr Mike J Smith
I recently came across LeoWorks an educational remote sensing package that is free for use in teaching. Developed by ESA and the Romanian Space Agency (yes they have one and obviously for a considerably longer time than the UK!) its not going to win any cutting edge awards but it is a very capable piece of software. It surprises me that there are no reasonable open source remote sensing packages (unlike GIS), but there you go..... anyway, it appears to be developed in IDL (which would make sense) and offers the following features (amongst many):
-split, combine, rotate, mirror, raster math
-stretching, histogram equalization
As you can see, surprisingly capable and certainly pretty much everything you would want to cover in an introductory module. And, to boot, it appears to run fine from a USB stick.