Marketplace
The Steam deal for this game is over but if you missed out you can now also get Limbo for free on the Humble Bundle store.
You will get a Steam key and also get a DRM free version in your library to download which makes this a great deal and better than the Steam only freebie in my opinion.
So definitely worthwhile getting even if youy already got the Steam deal that has now expired.
Enjoy.
Showing up as $9.99 for me.