Dot as indicator
Credit to Martin Pitt for suggesting this.
How to use it
Place a dot near an element, such a text entry in a list, to show that it has something associated with it.
When to use it
I don't think I've seen this used anywhere but calendars: Place a dot underneath a date in the calendar for every event that is happening on that day. This is an easy way to show which days have events happening, and how many. It could also be used as a menu highlighter, like other techniques on this website.
What to watch out for
It's important that the user know what a dot means, if you're using them to mark things associated with whatever the dots are sitting next to.