As discovered by Cult of Mac, Valve released Half-Life for Mac on Steam this morning, though the company has yet to make an official announcement about the product.
Other Valve games like Half-Life 2 and its episodes have been previously available on the Mac version of Steam, but users can now add the original Half-Life to their collections for $9.99. It has also been bundled into several different value packs.
Half-Life was originally developed for the Mac back when the Windows version was released, but due to disagreements with Apple, the Mac version was scrapped at the time.
The Half-Life page in Steam still indicates that the game is Windows only, but Cult of Mac bought and installed the game on a Mac to confirm that it functions. The game can be purchased via Steam for Mac for $9.99.