What are you trying to accomplish? What operating system do you use. The following list is mainly for Windows and Linux, though some can be used on Mac.
For general editing, Photoshop is a must and should be the first software you should have. If you're on a budget, the The Gimp is free. The Gimp is slower than Photoshop and the user interface is horrid. It does produce good results.
Irfanview (free), Xnview (free), and ACDSee (payware) are rudimentary image editors. They are most useful for viewing thumbnails. Highly recommend one of them.
http://www.irfanview.com/
http://pagesperso-orange.fr/pierre.g/xnview/enxnview.html
http://www.acdsee.com/
If you shoot in raw mode, then something to convert your proprietary raw images into 16-bit TIFF format. Canon may have a program to do this that may be in your software bundle, or you can use any number of raw conversion utilities out there. I like DCRAW (free).
http://www.cybercom.net/~dcoffin/dcraw/
Photoshop can do raw conversions also.
If you want to do panorama shots, Hugin is a very good image stitcher which is free. Takes a bit of time to learn, but produces better results than $300 programs. Read through the site and use Smartblend. Smartblend can make a lousy pano set into something quite passable.
http://hugin.sourceforge.net/
If you want to do HDRI (high dynamic range imaging)
, Qtpfsgui (free) does a good job, but will take time to figure out.
http://qtpfsgui.sourceforge.net/
There are many HDRI programs out there, so look around.
Exiftool (free) is invaluable for adding, changing and viewing EXIF and other headers. Take a vacation and forgot about the change in time zone? Exiftool can change time/date. Do a search for exiftool if you want a GUI for it.
Nconvert (free) is a invaluable program for doing batch conversions of images. I use this when I need to resize a bunch of images for web pages.
http://pagesperso-orange.fr/pierre.g/xnview/en_nconvert.html