Note: I only link to retailers that I have personally used, and am happy with.
That's no guarantee of perfect service of course, but it's better than nothing!

Let's Automate
Large range of HA gear. Supplied my ZWave devices.

Laser
X10 Specialists. Supplied my Barix Exstreamer.

Cyber Select
Supplied my X10 modules and the W800RF32 rf interface.

Dontronics
Australian supplier of useful electronics kits and components.

NetShop
Everything for your home network. Patch cables and structured cabling accessories.

Overclockers
Very competitive prices for computer parts.

Burn Media
Blank media and cases at very low prices.

TLC Direct
DIY supplies. Good for CAT5 and CT100 cable in bulk.

Screwfix
They're owned by B&Q so the money goes to the same place in the end, but at least you don't have to fight for a parking space.