Yamaha Grizzly 600
From Yamaha Wiki
The Yamaha Grizzly is a large utility all-terrain vehicle manufactured by the Yamaha Motor Company. It has a 595cc 4 stroke engine. Yamaha replaced the Grizzly 600 with the new Grizzly 660 using a larger engine that is 660cc derived from the Raptor 660 with a five valve cylinder head.
Beyond the engine, the biggest differences between the Grizzly 600 (offered in the US through the 2001 model year) and the Grizzly 660 (introduced in 2002), is the rear suspension. The older Grizzly has a ridged rear axle with a single shock. The newer Grizzly has independent rear suspension with dual shocks. This is reported to have significantly improved the handling characteristics of the newer model.
In 2007 Yamaha came out with the 700 Grizzly. As a option you can get ESP (electric power steering), that has been getting great reviews. One of the big changes from the 660 to the 700 is the 700 is fuel injected. Also, the 700 uses "gull wing" style A-arms on the rear, allowing for a shorter frame, and less weight.