Nebulosity: Not free, unfortunately (costs $80 currently), but is a very powerful software package for controlling cameras (mostly CCDs, but also Canon DSLRs) and stacking images. It will cope with field rotation if you don’t have an EQ mount. Also available for the PC. You can easily use Nebulosity without having to resort to Photoshop or other photo processing packages, so it’s worth the $80 if it means not buying Photoshop!
StarStaX: Very easy to use software, available for both PC and Mac, for producing star trail images. Simply load in the frames, plus any dark frames if you’ve taken them, and click the button to process. Very quick, with lots of advanced options to play with.