Inspirational garden design sites  :
Japanese roof garden  :       
Stunning design !
MyLandscape  Roof Gardens:  
High WOW factor in all the roof gardens !
Urban Roof Gardens :             
Inspirational pictures
Lofty gardens :                      
American loft gardens
Miller Roofscapes :                
Specialist UK company
Patio Garden :                       
Plan + instructions
Garden Design 1:                   
Common garden problems 
(too small/dark/steep/odd shape etc)
Garden Design 2 :                  
Various different types of garden
The Design Landscape Center: 
Has loads of info / photo's for inspiration
Inspirational Design :              
This site has a high WOW factor
Blue Angel Garden design
New York rooftop chic
Gardens 2wish4
Some London garden designs
G4D gardens
Ultra modern designs and a website to match!
Garden Design Andy Sturgeon
Roof, city, country gardens, lighting and sculpture
tGoD
Sustainable, contemporary info

Low level roof planting systems :
Roofscape :                        
Green technology for the urban environment
Greenroofs :                       
Their system even support trees !

Vegetable growing:
Growing from the roof up : 
Vegetable roof garden in Reading UK
Urban Agriculture :            
A thesis with technical insights
Vegetables in containers : 
 Pots or containers on the roof
Horticultural Perlite :          
Lightweight growing medium
Telegraph gardening :      
Veggies in containers
Hanging gardens of St. Petersburg : 
Russian veggiemania :~)

Other peoples gardens:
Sue Hayward :

Miscellaneous Sites:
Daily Telegraph Gardening Page
Huge amount of good advice
BBC Garden design
Virtual garden and inspiration
Wikipedia - "garden"
Huge resource - ever changing web page...
Wikipedia - "roofgarden"
Masses of info and pictures.

Technical- Structural
 Hard landscaping
European roofs :          
Things to take into account
The Decking Web :       
For fans (you love it or hate it!)
THE Paving site :          
The Ultimate UK paving site

Soil
C.H. Binder :               
 Lightweight roof garden compost


Raised planters : 
Home & Garden TV :    
All you need to know about raised beds.
Includes plans, info, tips...........

Plants for high places
My roof garden plants : 
What I've used on my roof !
My Patio plants
Close-ups of various plants
Crocus : 
Various plant suggestions
Themed garden borders 
$4.95 per plan !

Books
Amazon.co.uk :               A choice of books
The Anglian Gardener :    A selection of books

Watering systems
Hozelock :                 
Automatic watering system
Gardena :                 
Automatic watering system
Plantpal :                  
Automatic watering for pots
Leaky pipe:              
 Drips water slowly 
Crocus :                    
General info + shop 
Haws watering cans : 
The traditional method is sometimes the best! 
    
Odds and ends:
Architectural plants
Plants for 'difficult' places
Cat on a Hot Tin Roof:                  
Roof garden for cats :~)
Gabions 
Probably best NOT on the roof :~)
Lou Reed
Ode to rooftop gardens....

If you have any useful sites please let me 
know and I'll include them in the list.