This month a new game is being released called Still Life. I was viewing their website to see whether the game would be worth checking out when I stumbled upon their 'Prelude'.
Now, while it's not an ARG in the true sense of the word - at least, ARG fans will tell you it isn't - it still has the feel of one for the most part. It contains a series of logic, picture, code and number puzzles, and is quite entertaining. It's also quite addictive, trying to solve the puzzles to complete the day and move on, and to find out the next part of the story. It also apparently ties in to the main game somehow (which hasn't been released yet).
Anyway, if any of you enjoy puzzles and brain teasers, and have some time to kill, I highly reccomend checking out this game. If you know how to contact me, I can help you out with the earlier puzzles should you get stuck, but google search Still Life Prelude, and I'm sure you'll find websites with hints and tips, the forums which follow ARG games are your best bet.