Func reflective glass
Used to produce perfectly reflective glass that renders world + entities.
Typically, you want your glass brush to have nodraw on all non-reflective surfaces and you want to use a shader like lightmappedreflective in your material applied to the non-nodraw surfaces.
See hl2/materials/glass/reflectiveglass001.vmt for an example.
NOTE: Currently, you cannot use reflective glass in scenes with water, and you can only have 1 reflective glass in your view ( + pvs ) at a time.