My friend sent this video over about a new power assist device for wheelchairs called SmartDrive from Max Mobility. Check it out. I can't vouch for this or any other product I haven't tried out. Over at CareCure, they're saying its priced upwards of $6000, so its out of my price range but I sure would love to be able to use a manual chair.
The SmartDrive is a revolutionary new power asisst device that will change your life. It can power you up steep hills, through carpet and over grass. It is intuitive to use, just push to go and brake to stop, its that simple. It will go for days on a single charge. Plug in and get out there!