While Steam's store doesn't actually list a Linux icon for it, Valve announced on the official community hub for Half-Life: Source
that it's now available for Linux.
On the 25th of September Alfred noted this changelog
QuoteConverted Half-Life:Source to the new SteamPipe content system, for optimized delivery of the game
Added support for Linux and Mac OS X operating systems
Added an optional HD pack, you can enable this via the Video tab in the Options menu
Numerous bug fixes contributed by community members
Winner of over 50 Game of the Year awards, Half-Life set new standards for action games when it was released in 1998. Half-Life: Source is a digitally remastered version of the critically acclaimed and best selling PC game, enhanced via Source technology to include physics simulation, enhanced effects, and more.