Due to a fire retardant in the plastic LEGO is made from, it tends to yellow quite a bit over time — which isn't ideal if you're building a dock to match your white iPod. Blogger Smitty noticed this problem and created a site dedicated to re-engineering previously mentioned Retr0bright, often used for computer-case restoring, for use with his LEGO bricks.
The general process isn't too difficult; you'll just need a bit of hydrogen peroxide, glycerine, vegetable gum and some OxiClean. With those ingredients, you can create a gel that, when applied to the bricks and left out in the sun for a few days, will restore them to their original shiny colour. Hit the link for step-by-step instructions, and share your favourite uses for (restored) LEGO in the comments.