When I need drawing tools and other advanced capabilities, nothing beats Paint.NET.
Don't let the Web-oriented name fool you: this is a powerful image-editing program, one stuffed with Photoshop-quality features like layers, special effects, and plug-ins. The just-released 3.5 version of Paint.NET adds an updated user interface, new effects, improved startup performance, and a long list of tweaks and fixes. Best of all, it's absolutely free.
I'm perpetually amazed that you can get software this powerful that doesn't cost a dime. Consequently, I think using Paint.NET is a no-brainer for any user looking to edit, enhance, and otherwise modify images without spending crazy bucks on Photoshop. Bottom line: If you want to edit images, get Paint.NET.
Note: This program is donationware. It is free to use, but the author accepts and encourages donations towards further development.