Depends to what sort of level you want to go to. For simple
adjustments to brightness / contrast / straightening / red eye removal
etc then Picasa is as good as anything. For more advanced work then
Gimp is good but there's a learning curve to it ( same as any other
advanced program !! ) but once learned it's pretty powerful.

Two things to remember:

1) Quality of image cannot be improved - a low res snapshot will
always be a low res snapshot.

2) Work on a backup copy, not the original image