I use a photo editing program called Irfanview. You can download it for free (no nag screens about upgrading or anything like that either) from here:
IrfanView - CNET Download.com
I'm probably at risk of being laughed off the forums for telling you this, but I use Irfanview for a lot of Q&D edits when working with .jpg's because it's SIMPLE and FAST, while for heavier, more precise work on RAW files and such I break open Photoshop. Anyway, once you open a file in Irfanview you simply navigate to: Image/Color Corrections in the toolbar and use the sliders for the different color (R, G, B) corrections , the "Gamma" slider, etc. and then apply the changes when you're happy. There's a separate menu item for "Sharpen"; click it repeatedly to increase the sharpening effect; more than two clicks will promptly ruin most pics in my experience.
Furthermore, if you're going to use Irfanview, you'll want to download the "Plug-ins Pack" from the homepage:
IrfanView - Official Homepage - one of the most popular viewers worldwide
As well as the Microsoft Camera Codec Pack:
Download Microsoft Camera Codec Pack from Official Microsoft Download Center if you play with RAW files. Also, all of the above assumes you're playing on a Windows PC; if you're a Mac user I have no suggestions for you...
And now you know my dirty little secret!! Hope it helps.