I'd suggest OpenOffice.org Draw, available for download here,
http://www.openoffice.org/.
Here's a description It has a feature in which you can create your own custom gradients. You can create some 3d models, change the lighting on them etc. But it's absolutely free, no trial period or such, and it can create gradients and has all the basic editing features. And in the end, if you don't like it, you can always delete it. However, it sounds like it would be good for what you're looking for. Also, you may want to look into some other programs in addition to this one, based on your specific needs. Just a suggestion.
