This is a web site hosting the map pack played at JustinCon. It is occasionally updated with new maps (or refinements of existing ones).
Probably, you will want to follow these steps and install everything in one go.
If you are just interested in individual maps, review the gallery below. Click any screenshot to view a higher resolution version. Click the bold link beneath a map to download it.
You can also point sv_downloadurl to this page in an HLDS server, which allows clients to download at high speed from here, instead of at a slow speed from the game server.
You should probably get this too. It's a collection of all official Valve / Gearbox HLDM content released outside the Steam version. It includes things like the Multiplayer maps from the PS2 version, spraypaint from Half-life: Extra Data, additional player models given to Planet Half-life, etc.Download from ModDB (Local Mirror), unzip, and move the contents of the extracted valve/ folder into your local installation. See instructions above.