I have it on my desktop in a notepad file.
As for me, its on a CD, along with a receipt, email account details in case of recovery is needed, and a copy of Blockland v8 Mac (both a copy of my exact folder from 2008, and the DMG install).
I also did this when I bought Game Maker. And it paid off when I had to get more activation's put on my key after I had to reinstall Windows again. :u