Portal 2: Community Edition Beta is out now as a playground for modding

Portal 2: Community Edition has arrived now in Beta, bringing with it a whole lot of goodies for modders to play with. This could easily end up as the best way to play Portal 2, and to play and make mods for it with new advanced tools.

From the Steam page:

Portal 2: Community Edition (P2:CE) is a community-maintained version of Portal 2 that significantly enhances the way you make and play content, offering new capabilities such as a brand-new scripting system, an improved Hammer map editor and significantly increased limits over the original game.

Features previously thought to be impossible in Portal 2 due to the architectural limitations of the Source engine are now possible thanks to a wide array of improvements introduced in Strata Source, an officially licensed, heavily-modified version of the Counter-Strike: Global Offensive (CS:GO) engine branch.

These improvements include (but are not limited to) the addition of native DirectX 11 support, physically-based material surface rendering and a new Clustered forward lighting system with support for Volumetric Lighting. Existing engine limitations like maximum map bounds, entity and BSP lump limits have also been raised or removed entirely.

Performance improvements across the game have also been made in part due to the addition of a native 64-bit port of the Source engine, eliminating any and all game-breaking memory restrictions.

See their overview trailer below:

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With it being in Beta, you may encounter some issues.

As long as you own the original Portal 2 - you're good to go with it.