Unless indicated otherwise, sites are in English or one of its dialects.

Kites 

Artistic Single-line Kites
Kisa - German art kites and elegant photography.
Zoone - A playful gallery of art kites presented a unique style
Reach up and Touch the Sky - a slick artistic kite site about artistic kites 

Clubs and Shops 
San Diego Kite Club - the big local club, and a beautiful site
Kite Country - my local kite store, crowded with raconteurs on rainy days
Kites Spitfire -    some unusual European kites you won't see at your U.S. kite  store   
Kites in Italy  -   a big site with lots of links and photos of happy fliers

Fighter Kites
Cyberfighter website - an excellent site for fighter kite fans
Northwest Fighter Kite Club - a very active group of eccentric fighter fans.
Gerhards und Danielas kleine Kampfdrachenwelt - an inspiring fighter  site. Go there!
Fighter Kites - Dennis Icshe's fighter kites are favorites of mine.

General Kiting 
Nor'easters - Tony Heeschen's site about the Maine kite scene  Brrrrrrr!
Kite Jan - a fresh personal kiting site
Geert's Site - very finely-crafted kites. Inspiring!
Patrick Fulton's site -  KAP, tree houses, woodturning, and more fun
Ciel Libre - Didier Ferment's artistic ingenuity on display

Kite Aerial Photography
Cary Clement's KAP Site - a thoughtful site presenting aerial views of Maine and  tech info
Maxworthy's KAP - includes plans for beautiful kites.

Resources and Plans 
Vientocero - excellent Spanish site for planos y mas. 
The Kitez Search Site - the laziest way to find kite info on the web
Peter's Kite Site - plans for kites, and much more.
WordLingo Translation - a way to translate Dutch, since Babelfish doesn't
Bilboquet Kites   - An online kite shop with lots of info, forums and reference material
Wildkite - An inspiring new site for dual-line kite makers. Nice plans

Traction
NPW Kite site - a site devoted to the "Volkswagen" of traction kites and its evolution 
Foilmaker - with this impressive freeware, you can design the traction kites of your dreams
Dave Aberdeen's site - one of the Foilmaker guys delivers Surfplan on his fine site.
The Walk of Shame Buggy Club - a collection of kites to pull things across the earth's surface
Stuntvliegers Online - plans for the Bean, Kiwi and NPW5 kites, among others. In Dutch
Bozon Bean Pages - how to make the Bean traction kite
Stelios Alex - a ingenious collection of ideas for homemade power kites and gear
Frans Nijhuis website - a fine Dutch site for kite buggy building and riding  

Kite Discussion Groups

Rec.kites is possibly the most well-known kite group covering general subjects and commercial interests. There are other groups just for kite makers, traction kiters, fighter fans, NPW makers, boarders, buggiers and quad fliers that address more specific topics. 

Kite forums worth trying:
http://www.aerialis.com 
bilboquet.com 
http://www.intothewind.com/infoex/
http://www.kitebuilder.com 
North American Fighter Kite Association (Topica.com) 
http://www.kiteconnection.com

Landsailing

R/C Sailcars - Home of the Windjammer R/C landsailer. Quite a nice rig!
Colorado Land sailing and Ice boating -  Rick Hypes' site is a must for landsailer builders.
NALSA - the big U.S. land sailing organization
The Sirocco Land Sailor Association Home Page  - for pilots of the Sirocco Land Sailor, and items of interest to sailors of other types.  I know, a sailor is a person, a sailer is an object. Oh well..

Boomerangs

Bailey Boomerang Catalog  - a big online catalog
Boomerang Technique - how to throw them
Boomerangs -  a large boom site
Bumerangs - huge selection of plans and info 

Boating

The Cheap Pages - boating cheap and other subjects.  I love the title
The Mother of All Maritime Links - The best of its kind, everything nautical
Traditional Skin Kayaks - The original way to kayak

Reference sites

Sewing 101 - A useful source of sewing information geared to kite making.
Sewing with Debbie Cosgrove - Close-up pictures of thread.  Of interest if your machine is driving you nuts. Much more on machine sewing
The Lock Stitch  - The best place to find info on that old sewing machine you ended up with.  
Sailrite Sail Supplies - lots of helpful sewing machine information, and for sewing heavier fabrics
Wind Scale Conversion - how strong is the wind?  

Tools and Supplies

Campbell Hausfeld - low priced welding and other tools
Harbor Freight Tools  - tools for the bargain hunter
Rockler (formerly The Woodworker's Store) source for slippery nylo-tape, Baltic birch and many other very nice things.
Hang em High Fabrics - a super kite supplier, with excellent on-line service, and low prices 
Kitebuilder - a source for kite making supplies