You may have played the Half-Life 2 mod Minerva when it was released a few years ago, but now it's back, and enhanced, and on Steam, and totally free.
Minerva is the production of Adam Foster, a designer who got his current job at Valve by making the surreal mod. Says Foster (in an e-mail to Kotaku):
It's taken long enough, but via lots of nagging and prompting from fellow Valve employees I've finally got round to getting MINERVA, the Half-Life 2 mod which got me a job at Valve, up on to Steam. It was originally released in late 2007 to pretty much universal acclaim, but now there's about to be a super-fancy Director's Cut edition with tweaked visuals, bug fixes, better puzzles and all kinds of subtle improvements. Nothing hugely new, just old stuff tidied and polished for this re-release.
Expect the usual single-player shooting and puzzles, a healthy dose of mystery and plenty of on-screen dialogue from the unseen Minerva, who guides (and insults) you on your journey through a vast Combine underground facility.
It requires Half-Life 2: Episode One as a base - but once you've got that, the mod adds quite a few hours of free, arguably Valve-quality gaming.
You can check Minerva out on Steam right here. Because free games are awesome!