As part of my little Automator screencast I did earlier this week, I used a special symbol in the file name. The character viewer opened during the screencast and I received several emails/tweets asking about how I did that.
A lot of people don’t even realize the character viewer is a thing. You can activate it in the Keyboard Preferences by ticking the “Show Keyboard & Character Viewers in menu bar” box and the character viewer icons shows up.
Clicking Show Character Viewer opens a screen we've had for some time in OS X that lets you pick among the various symbols not otherwise easily accessible.
Starting with Mavericks, Apple made this even easier. You can open a slightly friendlier (more iOS-y) character viewer with the key combination Control-Command-Space.