I've just published a free online music theory tool, the Interactive Circle of Fifths: http://randscullard.com/CircleOfFifths
I look forward to feedback from the readers of this forum, since you folks are exactly my target audience for this tool. I hope you find it useful and interesting!
(Be sure to check out the in-depth User's Guide. Even if you're already familiar with the circle of fifths, you may find some new ways to apply it in your music.)