It's a new version of XashXT, an extended front-end for Xash3D engine. It's compatible with latest build of Xash3D (2153) and provides full support for new extended map format, BSP31. It also has some bug fixes and some new entities (read the updated documentation inside for details). Click "read more +" to see some important notes about installation of XashXT.
XashXT (aka Xash Mod) it's a MOD for Xash3D Engine with a set of demo maps, additional tools and documentation, and it should be installed as mod in addition to fully working installation of Xash3D Engine + Half-Life game. It can't work properly without Xash3D Engine files or without some resources from Half-Life. So at first make sure you have done this: Moddb.com before you try to install XashXT. Then extract all XashXT files from XashXT_061.7z in your Xash3D game directory. And run at least once hl.exe (main Xash3D executable) before you run xash.exe (Xash Mod executable). Because this action is necessary to set valve directory as parent directory for XashXT. Otherwise some Half-Life resources will not be used when you run Xash Mod itself.
If you already have this problem (missing sounds or textures when you are running Xash Mod), you have to open gameinfo.txt in xash folder and set basedir parameter to "valve".
P. S. If you need "portable" version of PhysX libraries for realtime physics simulation in XashXT (if you stlii don't have NVidia PhysX driver installed on your PC), it's here. Just extract those files in the main Xash3D game folder or in Xash3D\xash\cl_dlls folder.