Dropbox
I wrote Dropbox their first investment check on June 26, 2007. What strikes me as I look at a copy of the check is that Drew and Arash must have been fairly effective at getting all their incorporation paperwork completed. I often had to chase startups well into July.
Little did I know back then what a momentous check this one would be: that 11 years later, Dropbox would be the first Y Combinator company to go public.Congrats Dropbox
While I don't think that Y Combinator's agenda is to make money or take companies public, this is a great example that playing the long game is what pays off in the end. If this is the first of many to come, Y Combinator is in for a very prosperous future.