Steam News (thanks Richard and Frank) has word that the Lost Coast HDR level for Half-Life 2 is now available. This page can help you figure out if you match the level's system requirements, and here's word:

Half-Life 2: Lost Coast is now available for play for all Half-Life 2 owners.

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.