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.