Download some PNGs. Put them in the iOS simulator app bundle. Rinse and repeat whenever Xcode updates the simulator.

Lossy compression for 24-bit PNG images. That is, it'll take your 24-bit image with alpha mask and drop it to an 8-bit image – keeping the 8-bit alpha channel.

Useful compendium of icon representations of various metaphors.

3D CSS. Wow. Just wow.

Very useful ems calculator, allowing you to specify root font size, and calculates ems for nested elements on the fly.