Ducati:
A jam-packed site for all Ducati aficionados.
http://www.ducati.com/

Harley Davidson:
A very professional site, full of animation and easy to navigate, you can even find out what's on the menu in the Harley Cafe in New York.
http://www.harleydavidson.com/

Honda:
Slick and full of information - look out for the forthcoming UK site.
http://www.hondamotorcycle.com/

Kawasaki:
Overall, it's one of the least impressive sites in the category.
http://www.kawasaki.com/

Suzuki:
A good site, though it's a shame about the lack of UK content..
http://www.suzukicycles.com/

Triumph:
It's quite slick, but it's not one of those sites that has endless animated graphics an special audio effects.
http://www.triumph.co.uk/

Yamaha:
As it stands, this site is almost entirely product based, with the news ticker style Hot Topics section the only exception. One other useful section is Buzz Clips, an archive of Yamaha news pieces/releases.
http://www.yamaha-motor.co.jp/