Editing your photos for stock can be as simple or complicated as you want to make it.  Many people submit photos directly from their point and shoot camera, however you will see best results in both acceptance ratings and sales if you edit your photos.

Free Editing Software

Picassa: A nice simple program from google for editing and organize photos.
Gimp : Probably the best free editing software.  It is a bit complicated to used if you are new, but worth trying out if you are determined and can't spare the $$ for an adobe product.

Cheap Editing Software

Adobe Photoshop Elements: A great program that is very close to it's big brother Adobe Photoshop CS3.  This is the software I would recomment if you are not going to pay the big bucks for CS3.  It will get you used to the photoshop layout and provides tons of editing options.  If you learn to use this program you will be well on your way.
Pixelmator: I can't say I have tried it but it looks like a nice program for the mac.

Professional Editing Software

Adobe Photoshop CS3: Only one serious choice here really.  Learn to use it and the power will be in your finger tips .