http://www.nomination.com/
I bought one of these today for my mum for Mother's Day. And she bought one for me for my birthday!
I'd never heard of them until I saw a friend wearing one on Monday.
They are quite cheap compared to Pandora (and very different). The standard bracelet was only £11. The links range from £18 upwards.
You buy the bracelet and then buy the individual links. As you buy a new link, one plain link is taken out which you keep. Then when you have no plain links left in your bracelet because you have replaced them with the charm links, you have a plain bracelet to start all over again with!
I shall wear mine on the same wrist as my Pandora - they look quite nice together.