Even if Valve isn't planning to make Half-Life 3, or even Half-Life 2: Episode 3 anytime soon, some Valve staff are still keen to discuss it and help people play around in that shared Valve universe a little more. One ex Valve employee, Cayle George, has released a brand new Half-Life mod that opens up different aspects of the world.
Called Half-Life: C.A.G.E.D., it's a prison escape mod, that charges players with escaping a facility using their wits and a variety of weapons. It's not the longest of experiences, but it is brand new Half-Life content from an ex-Valve employee, so it's basically royalty. It also comes with a commentary track and its own soundtrack from LazerHawk (thanks PCGamer).
The whole thing has a very retro feel to it thanks to the lighting choices and visual style George opted for, though using VHS style tracking glitches over the trailer and a very synthy backing track certainly contributes to that too.
Although this isn't the Half-Life mod to end all mods, it's received mostly positive reviews so far, with a number citing good level design and the soundtrack as major highlights. It's not too surprising that it's come out well though. Afterall, George was a designer on Portal 2 and Team Fortress 2. They know what they're doing.
