this is a bit of a vague question but I have been building my own pi robot and am just getting to the stage where I wish to start adding some sensing capability however I am not an experienced techie and wondered if here were any really good sites that you experienced lot visit? At the moment I am relying on amazon :p
If you go to the Raspberry Pi site (http://www.raspberrypi.org/) and look at the sponsoring companies (Currently under "Buy a Pi" on the main page), you'll notice that most are electronics distribution companies from around the world.
The reason that the RPi is so affordable is because they partnered up with these companies to help subsidise the component costs.
If you want a "kid in a candy store" experience with sensors, go to those sites. You'll be overwhelmed in minutes. Not only are they reliable suppliers (though I can only speak from experience with element14 and RS) but you are helping the companies that invested in the little box of joy that is the Raspberry Pi.
I know of two great sites for supplies and also for information on how to use external hardware with the Raspberry Pi.
You should definitely check Adafruit's Raspberry Pi Tutorials, where they teach you how to connect a number of different hardware to the Pi.
Spark fun also has a number of tutorials, but they don't seem to be as specific to the Raspberry Pi as Adafruit's. But you should give them a look also.