Photo courtesy Valve

Valve’s mind-melting sci-fi puzzler Portal 2 has hit Steam a few days early after ardent fans raced through an ARG and pumped countless hours into a selection of indie games.

The downloadable games, bundled in the budget-price “Potato Sack,” were all embedded with secret Portal-themed levels, music and secret messages. Plus, each game featured a few unlockable potatoes. We’ll come back to those secret spuds in a moment.

After fans decoded the entire enigmatic riddleverse — which involved deciphering Morse code, archiving deleted blog posts and de-encrypting coded audio files — they ended up on the website GLaDOS@Home. Here, the series’ menacing AI superbitch GLaDOS promised to unlock Portal 2 early if enough gamers dedicated their CPUs to playing indie games.

The site parodies crowdsourced citizen science apps like Folding@Home — which lets you lend your console’s monster nine-cell microprocessor to researchers studying diseases such as cancer — and Seti@Home, which borrows your CPU strength to analyze radio telescope data from space.

In the end, Portal 2 came out (in the United Kingdom) late April 18, 2011. It’s good for those of us in the U.K. who would have had to wait until the following Thursday or Friday for the game to unlock, but a rather bitter pill to swallow for gamers in the United States who would have been treated to the game a few hours later, regardless of how much time they pumped into GLaDOS@Home. (Portal 2 was released Tuesday in North America.)

A few upset fans, who spent hours playing games like Audiosurf and Amnesia for the early release, have taken to forums to complain about the negligible reward offered for their time. The vast majority, of course, are too busy playing Portal 2.

Still, while the ARG’s outcome might have been a little anticlimactic, there were a few surprises in store on the morning of release. If you unlocked the 36 potatoes that were bundled in the indie games before Portal 2 came out, you were treated to a goodie bag of Valve games: including Half-Life 2 and even Portal 2 itself. Around 1,800 fans took the time to find all the spuds.

There’s also an unexpected inclusion in the PC and Mac versions of the game: an interactive trailer for J.J. Abram’s new mystery flick, Super 8. Valve and Paramount remade a clip of the upcoming movie in Portal’s Source engine, letting players survey a train’s wreckage before being whisked to the film’s official site.

Whether you took part in the ARG and GLaDOS@Home or not, Portal 2 is now available to download on Steam. It’ll hit Xbox 360 and PlayStation 3 later this week. Our review of the game will hit the site very shortly.