The Smart ForTwo is a fantastic vehicle. You may have seen them running around recently here in the US, but they've been all over Europe for around a decade. They debuted in the US to a waiting list of around 30,000. Not bad!
I'm a custom car freak, and I nearly wet my pants when I came across this modification to the Smart. It has a rear mounted 3 cylinder engine, which is easily removed and replaced with motorcycle engines. I've seen variations with GSXR 1000 and Hayabusa engines. Its been pushed to a horspower of about 180hp (compared to stock Smart 60hp). Zero to 60mph in around 4.2 seconds, 1/4 miles in 12.4 seconds. They call it the 'Smartuki'. The speed of a bike, the weather protection of a car, plus very easy to park in the city. And all for under $20,000 in modification costs!
See video below:
Here's one beating a Ferrari 430 :)
I saw quite a few of the Smart Roadsters while in Europe, these things look like mini Dodge Vipers. They were discontinued in 2005, but Project Kimber purchased the tooling and it seems they will be reappearing in the coming years. They are amazing little sports cars, see photos below.
I came across an image of the jeep-like Smart Crosstown online, but haven't heard or seen much about it.