Biorhythm
A pseudo-science that's at least an interesting concept, the chart could be useful for roleplay characters or just to look fancy.

Countdown
A tool to make an image that counts down to a specified date and time. Mainly used for keeping track of when a game/program is supposed to release.

Countup
Counting how long it's been since a certain moment. Used for anything from "We've been a couple for this long" to "x days since last science lab accident".

Time
Trying to coordinate roleplays or online gaming or something else where time zones could be an issue? This will show other people the current time where you are.

Sources
The source code for all of the above.