Eagle Eye ARG
I can't say enough good things about this.
A really compelling ARG, finishes in 10 minutes, and a complete single player experience.
Really - if you've got 10 minutes, sit yourself in front of a computer and head over to http://eagleeyefreefall.com/
You'll need a phone handy.
It's amazing how well this makes the experience real. I found myself looking at my phone afterwards, wondering what was going to happen next.