These are so cool. They bring a little something extra to those police car, robot and cityscape Lego creations. Nate is a Lego purist (seriously, once we had to pick through a box of Lego to cull the off brand blocks from the herd) but even he has requested a set of these - which are compatible with all major brands of interlocking building blocks.
They are $9.99 for two bricks and are available in green, red or blue, each paired with a white brick. You can find them at ThinkGeek.