Free software :
Picasa - easy; Good photo management; very dummy proof; very easy to undo; does not affect your originals
More advance (ie. photoshop type editor)
Paint.net - small and fast. Very powerful once you load in the plug-ins that you need.
If you are paying, then :
Lightroom - easy enough; good photo management; same as picasa above, but a bit more powerful and better with curves and other advanced adjustment features.