Nyx

Download Nyx.exe (the standalone app) and run it. Using Millennium? Take Nyx-millennium-installer.exe instead.
New to Nyx? Read the README first.

Nyx adds games to your Steam library from free manifest sources, straight from the Steam client: an Add to library button on store pages, DLC unlocked together with the base game, and known fixes it detects for your games and can apply the compatible ones. Settings live under Steam ▸ Nyx (or the tray icon), in English plus 18 languages. Nyx keeps itself up to date from this release.

_{Millennium users: re-run the installer to get the latest plugin.}