Metro 2033 is free to download on the Humble Store. And this isn't just a free trial, as you can keep the game indefinitely.
This offer is valid until 10am PST (i.e. 6pm UK time) tomorrow, 8th November.
Furthermore, the Metro Redux Bundle - which contains enhanced versions of both Metro 2033 and its sequel, Metro: Last Light - is half off for the next four days, bringing the price from £33.99 to £16.99.
Digital Foundry found Metro Redux to be a stellar upgrade of an already great series. "What's surprising is just how well these games hold up compared to modern day next-gen releases. Combine the upgrades with the enticing price-point and it's a compelling proposition for both newcomers and console double-dippers," Richard Leadbetter wrote in his Metro Redux face-off.