Elevator: Source reminds me of a time back when technical limitations were more constraining, before some of today's popular genres even existed. A team could build a game about inner tubes, or put a banjo-playing man on a box for a shmup, and that was perfectly acceptable behavior.
That freedom still exists today and is tapped into by independent designers, though some would argue that the lack of limits on creativity can be paralyzing. With mainstream game budgets as monstrously huge as they tend to be, it's no wonder publishers aren't jumping up and down to fund risky projects.
This is all a long-winded way of saying Elevator: Source is batsh*t crazy on a conceptual level, and I love the game for it. You'll need both Garry's Mod and Half-Life 2: Episode 2 to partake in this delightful co-op adventure -- otherwise, give the heartwarming video demonstration above a watch.
Today’s Greatest Thing Ever: Elevator Source [Rock, Paper, Shotgun]