There are a number of Radio Controlled cars available that have been made more accessible, such as the switch adapted Hitari Tumbler above. Beyond the fun of driving them into people's legs, they can be used to play 10-pin bowling with tin-cans, have the wheels coated in paint for art-creation, set-up for races and stunt courses and so on. Read on for more...
Enabling Devices have loads of adapted RC toys, such as their one-switch Disco Bus, two-switch Go Go Bus and four-switch Billy Fun Wheels. All seem a lot mellower than the Tumbler so great for those who'd appreciate something a little more sedate.
A new company to me, Adapted-Toys seem to have a few fun looking one-switch toys at decent prices. This stuff is firmly in the area of people fancying taking on the Do It Yourself route.
For more inspiration to go the Do It Yourself route this three-wheel adapted Turbo Twister stunt car is probably the most eye-catching of the lot.
And finally, the iPad compatible Switchamajig offers a way to make some cars more accessible with a really useful system for combining actions, such as forward and turn to a single touch-senstive area.