OpenAG is an open-source client of the Half-Life promod Adrenaline Gamer, completely rewritten from scratch on latest Half-Life SDK. It adds new features (see list below), bugfixes and other tweaks over the original mod, while maintaining the ability to play on all currently existing servers.
To play it, you need to own Half-Life on Steam. Just download the full package and extract the contents to your Half-Life folder. OpenAG should show up in your games library after you restart Steam.
If you already have Adrenaline Gamer installed, it's enough to just replace the old client.dll file with the new one in the Half-Life\ag\cl_dlls folder.
Most important features present in the vanilla AG client—HUD elements, commands and so on.
Fixed mouse input ensuring a pure raw input experience. Make sure to use m_rawinput 1.
Lots of bugfixes.
Native Linux support.
Ability to force playermodels for each player or team.
Built-in perfect autojump, as well as a replacement for _special.