I got bit. :-/ I installed "Youtube Downloader." for what appeared to be a good and valid reason, really it was.
Then, today, when I was attempting to work on a page for a client, I saw a weird javascript in my source that started:
var hkghawgalkgklrgjlargjsrhg_hejrghakwghakwegkawefak = (function(){ var e=null,f=[[15,18,7,19,2,0,17,14,6,3,11,20,16,1,9,4,8,13,12,5,10],[19,1,16,5,14,17,13,18,15,8,2,20,7,11,4,9,12,10,3,6,0],...... (lots more)
What's weird is that this was getting injected on a page that I had on my c: drive.