I am new to this site and thought I would share an R/C Lawn-mower project I just finished. I know this is not a new idea, but it was a very helpful learning experience for me, as well as being extremely useful.
I used an Atmega168 microcontroller and came up with the code to translate 2 r/c servo signals into forward/reverse PWM values. Then I designed and built a dual H-bridge to use as the speed controller for the bot. I also added a 2nd Atmega168 to separately decode a 3rd servo signal connected to a toggle switch, which I use as a failsafe and remote shut-off switch, disconnecting all power to the motors in case of a problem or signal loss.
The power comes from 2 wheelchair motors using #25 chain to transmit power to the rear wheels. It uses 2 deep-cycle marine batteries for optimum run-time.
For more information, check out my website. I have most of my projects listed there.
Here is a video of the Lawnbot400 in action: