Make your color figures accessible to the colorblind
According to Wikipedia, as many as 8% of males and more than 1% of all people have difficulty distinguishing colors. Thus, when designing presentations, figures, and user interfaces, scientists should try to keep things accessible. Color Oracle and Sim Daltonism are free software utilities to simulate how the colorblind might see your artwork or figures.
Both utilities simulate several types of color blindness, and both offer keyboard shortcuts to switch between the various simulations. Color Oracle is controlled from the menubar, whereas Sim Daltonism lives in the Dock. A potential disadvantage of Color Oracle is that it only allows static full screen viewing of the color simulations; as soon as you click with the mouse or type, the simulation ends. Sim Daltonism instead displays the live color simulation in a floating window, with the simulation centered on your mouse pointer. Thus, Sim Daltonism might be better for certain applications, where things might be dynamically changing in response to user input.
Overall, both applications are great pieces of software that can make figures, websites, and presentations better by making sure we keep them accessible to the color blind.
On my website, I have a couple screen shots, but the best way to get a feel for these applications is to download them and test them yourself.
Both software links above were found via Daring Fireball.