A while ago, I decided to try dropbox. Since I could apt-get it, I thought it was free software.
However, I quickly discovered that the apt-get command only downloaded a downloader for dropbox daemon.
So I stopped that dead in its tracks, did an apt-get remove, aaaand...
wait a second.
The repository won't go away! In fact, if I apt-get update, it's still in there:
Ign http://linux.dropbox.com wheezy InRelease
It's not in my /etc/apt/sources.list, so how did it get there?

apt-get install etckeeper, pick your favorite version control software, andetckeeper init. This way your/etcis automatically under version control, and you can easily find out when this appeared, and from there why. – Gilles Jul 26 '12 at 23:25