Some of the sites I enjoy:

Swedish BMW motorcycle club (in Swedish):
http://bmwklubben.se/

The biggest adventure site, can be a bit crowded:
http://www.advrider.com/

Great UK site for BMW GS bikes:
http://www.ukgser.com/

Small but good r1200gs site:
http://gsri.info/

Great Suzuki v-strom site (I use to have one):
http://vstrom.info/