It's hard to beat free but when you want more then the basic gear design free software offers, look into Gearotic Motion, seems it is $150usd now, it goes way beyond basic spur gears and once purchased any further updates or enhancements are no charge ever.
Art is continually expanding and updating the soft ware and is now barely recognizable from it's first generation, there is also a user forum where Art/Bob or others are quick to help.

They also have a demo version.

"Gearotic Motion Gear design Software"


Check out YouTube for Gearotic Motion as well for a lot of vids and demos



Ken