Requires Portal. So you finished Portal a few times. You even did all the challenges. You’d like to think of yourself as something of a Portal pro. Well, you might want to hold off on the self-congratulation until after you’re taken a look at Rexaura. It’s a super hard Portal mod that includes a fat new 20-level campaign made for puzzle-hardened veterans. The new levels make extensive use of energy pellets and even include new gimmicks. However, one of Rexaura’s most pleasant aspects is the fact that the puzzlers are made to be tough in their deduction, not their execution, so it’s easy sailing once you reach your “eureka!” moment. Add Steam achievements, new voice acting, and some genuinely funny new quips, and you have a formula for a winner. Also, GlaDOS now sounds British. How cool is that? Check it out over here.
Requires Half-Life 2: Episode Two. As a jaded and cynical old gamer, I don’t get creeped out or scared easily. But holy cow, Grey comes close to doing just that, more so than any of the mods on this list. Shucks, even commercial games could learn a thing or two from it. Borrowing many ideas shamelessly from virtually every horror game under the sun, you can expect loads of puzzles, highly grotesque enemies, and an endless supply of dark rooms and corridors. Grey‘s most saving grace is the extended play value: complete it to unlock assorted challenges and a “hard” mode, with a remixed campaign, more enemies and a “proper” ending. Creep on over here to get it.
Requires Alien Swarm (shown below). Another testament to the flexibility of the Source engine, Lambda Wars takes the familiar Half-Life 2 premise and turns it into an RTS. Players opt between Combine and Rebel forces and duke it out in assorted locales in and around City 17. It’s still in beta, but there’s already loads of content with a pretty solid core backing it up. Perhaps most similar to the Warhammer 40K: Dawn of War series, it’s relatively simple compared to other strategy games, though it nonetheless remains polished and engaging. Move on over here to add it to your library now.