Paint.net is my go-to. Simpler UI than Photoshop, and a free download. Unlimited undos/redos for those prone to fucking up on design stuff (like myself), and a easy to learn interface for those who aren't professional graphic design types (seriously, everything I need to do, it's completed in the LOGICAL way through this program; basically the exact opposite of what I hear GIMP is like).
Bridged the gap between 15 years of using MS Paint and something that is a bit beefier, for me.