tools

If you work in a noisy office (or home) environment and can't concentrate, this is a must-have Mac app: http://www.blackholemedia.com/noise/

Really nice and useful tool:
http://www.pixelatedsoftware.com/products/pixelstick/

Tools like this one are plenty, but this particular one has a great user-interface. Better than any other free one, I have seen.