A project called Half-Life 2: VR has recently been added to Steam Greenlight which, as you can imagine, lets you explore Valve's classic game through the medium of VR.
The project, in development since 2013, is compatible with both the HTC Vive and Oculus Rift headsets, and offers motion controller support, updated effects, maps, models, and textures, realistic weapon interactions, and more.
You need to have the normal desktop version of Half-Life 2, as well as Episodes 1 and 2, to use this mod, and if you're interested in hearing more about the development process, be sure to head check out the Road to VR article linked on the mod page. Which games also need VR mods?