Hardware Suppliers
Sheepwalk Electronics
There are various Dallas Maxim 1-Wire interface adapters and sensors available from Sheepwalk Electronics. Some kits are available unassembled.
Ciseco
There are various Pi-related cards and accessories available from Ciseco. Assembly instructions can be found here and the support forum is here.
Velleman
Velleman do not supply any Pi-specific boards or accessories but they do sell generic USB devices. I have used the K8055 and K8064. Their website is at Velleman. Most boards can be bought either assembled or in kit form.
RS Components
One of the two main-stream suppliers of Pis. RS also sell every conceivable component that you might need. Can be pricey but if you have an account, next-day postage is free.
Farnell
The other of the two main-stream suppliers of Pis. Farnell also sell every conceivable component that you might need.
ModMyPi
Supplier of well-made Pi cases and other accessories (GPIO, USB devices, SD cards, etc.).
Quick2Wire
Supplier of Pi interface boards. Not yet tried out.
Pi-Supply
Supplier of Pi intelligent power switching kits for your Pi.