According to SteamDB, the buzz surrounding Half-Life 2's 20th anniversary has brought a large number of players back to the single-player first-person shooter: at the time of writing, its peak 24-hour concurrent player count reached 52,029, which means that more people are playing Half-Life 2 now than at any time since Steam began recording concurrent players in 2008 (or at least since the data was available).
It should be noted here that this data misses the initial release of "Half-Life 2" in 2005 and the release of "The Orange Box" in 2007. Even so, this is still a pretty impressive performance for a 20-year-old single-player game. While pure nostalgia and the buzz generated by Valve's two-hour anniversary documentary are undoubtedly important factors, the main driver may be Valve's big new update for Half-Life 2.
One of the biggest highlights of this update is the addition of three and a half hours of developer commentary to the game. Valve has always been a leader in the gaming industry in providing in-game creator commentary audio tracks that players can access at various points in the game levels.
The update also notably integrates Half-Life 2: Episode One and Half-Life 2: Episode Two, as well as the Lost Coast demo levels into the base game's launcher. The last major update introduced support for the Steam Workshop for modding - which will hopefully give the venerable Half-Life 2 modding community a shot in the arm - but the Anniversary Update also includes a series of minor fixes and graphical tweaks.
In addition to updates and documentaries, we've also seen the re-release of old Half-Life 2 development footage that was once thought to be lost, as well as news that Valve will release an updated and expanded Half-Life 2: Raising the Bar artbook.