The source of a download!
I sometimes find files that I downloaded weeks ago and wonder where I got them. Well, it turns out that OS X has this feature built-in. You just have to select the file and hit "Command-I". It will display the info as usual. You can then click on the arrow at the left of "More Info". The "Where from" option will appear with the URL of where you got the file. The only thing I noticed is that it seems to work only when you download the file with Safari. It doesn't look like Camino or Firefox records that info.