Who, Where, When is a web application I wrote back in 2001 when I was traveling frequently and so were my friends. I wanted to make sure we didn't unknowingly miss a chance to cross paths in a foreign land.
These days, it's pretty much rendered obsolete by calendar sharing, which is way more general purpose anyway. So I make the source available simply because it's there.
Heck, it's probably still running, so you can try it out if you want to see how it works.
The source is on Github: samskivert/whowhere.
Who, Where, When is released under the GPL.