I think the most popular is the Honda Monkey in Britain. (visit http://www.monkeybike.co.uk )
I had a bunch of Monkeys, daxes and gorillas
but these were my last projects:
¤ my ex-Honda dax:
first stage:

second stage:

¤ my Tuned Honda Gorilla:
Specs:
- Takegawa 124cc r-stage
- Daytona 5plate clutch
- Kitaco 5 gear
- Keihin CR26 Special carb
This gorilla has got a top speed of about 80mph, wich is very fast for cush a small bike

Thanks for watching
grtz