Valve has given a double thumbs up to a fan made Portal 2 campaign called Aperture Tag, which sees players taking on the role of a brand new test subject, to provide new results about the gel guns, which featured in the latter half of the original Portal 2 campaign.
In the 27 level campaign - which you can play through by yourself or with a friend, though the developers have said cooperative play is a little buggy right now - you'll meet new characters, face new challenges and have to complete a different set of puzzles without the usual portal gun. There will be portals present mind you, but your abilities will extend only as far as shooting multi-coloured goo all over the walls.
Aperture Tag was inspired by the original team that created the gel gun concept for Valve and was later hired on to help develop Portal 2 at the company. Valve has now given the same sort of approval to this campaign, allowing the team to sell it through Steam - though they don't work at Valve, yet.
The campaign is currently priced at $4.89, down from the ongoing $6.99.
Along with Steam integration, are all the added features you would expect, including cloud saves and trading cards. There's also Steam Workshop support, so levels can be added at will and there are already a few extra ones for those that blow through the campaign in short order.