Rhododendron Species Foundation
Links

R. irroratum 'Polka Dot'

Hardy Fern Foundation
University of British Columbia Botanical Garden
Vireya Site
An excellent reference site for Vireya Rhododendrons
Vireya Vine
Bovees Nursery
A Vireya specialty nursery.
Vancouver Rhododendron Society
Contains a Links page to many Rhododendron related sites.
WNARSP
Western North America Rhododendron Species Project
Greer’s Gardens
Harold Greer is a very knowledgeable plantsman – he literally wrote the book (or at least one of them, and one of the most useful, at that).
Hennings
Steve Hennings' site with many links and his own Rhododendron and Azalea Bookstore
Glendoick Gardens
Peter and Ken Cox, run a garden and nursery in Scotland. Worth a browse through their catalogue, though you may not be ordering many plants from afar
The Lake Tapps Rhododendron
The Lake Tapps Rhododendron Garden and Nursery in Washington State, has a searchable catalogue, and some scenes from their garden.
A Sandy Rhododendron
A commercial site with many pictures and an interesting ‘Meta Tour’ – if you don’t want to search for a particular plant, hit that button and it will bring up pictures at random – you get to see hundreds of rhodos, for a few seconds each. Talk about a screen saver! You have to be quick if you see one you don’t know and really like the look of, so you can stop it and read the write-up
The American Rhododendron Society
Royal Horticultural Society
The site of the Rhododendron, Camellia and Magnolia Group of the RHS
Azalea Society of America
Some of this site is limited to members, but much of it is open for readers and it includes material on care and cultivation as well as a free newsgroup that you can join whether or not you are a member of the Society.
Seattle Rhododendron Society
Newsletter, rhodo of the month, and educational material
Milner Gardens
Meerkerk Gardens
Located on Whidby Island, in Washington State, this site has some nice pictures and references.