Valve has announced that the "Lost Coast" single-player level has now been released.
"Half-Life 2: Lost Coast, a single-player level custom-created to
showcase High-Dynamic Range lighting (HDR) in the Source engine, is
available now to all owners of HL2. As a technology showcase, Half-Life
2: Lost Coast's system requirements are very high. During installation,
Steam will prompt you if your system does not meet the recommended
configuration. We hope you enjoy it."
Related Link: Steampowered.com