Official:
news from me - Mark Evanier's blog about TV, movies, theater, news, politics and other forms of fantasy
POVonline - Mark Evanier's homepage with tons of information
Groo Stuff - a section of POVonline devoted to Groo
Sergio Aragonés - Mr Aragonés own webpage!

Fan sites:
Groo the Wanderer - Groo in the Internet, a place with mixed information about Groo.
Join the Groop - If you're not a member of the Groop here´s where you apply
The Norwegian Groo Page - lot's of pictures and information on this page. Also contains reviews of international Groo comics.

Other:
Marvel Comics - Groo's second publisher. Marvel published 120 issues of Groo and some reprints.
Image Comics - Groo's third publisher, published 12 issues.
Dark Horse Comics - Groo's current and current publisher which also releases some reprints.
Groo at Wikipedia.org - Groo featured on the Wikipedia
Groo's own Wiki - amazingly the server has not been sunk yet