Team Fortress 2 Gold Rush update is now available for free download via Steam. The update includes the Gold Rush map with its own gameplay style dubbed Payload. Perhaps more important than the map, this update marks the first set of new achievements and unlockable weapons.
The first class to receive the update is the Medic, who can earn three new weapons -- Blutsauger, Kritzkrieg and Ubersaw -- obtained by earning 12, 24 or all 36 new medic achievements, respectively. All the new achievements, including a complete Medic icon set (PNG files) can be found at the Gold Rush site.
Update: It's also worth noting that this weekend, May 2 - 4, Steam users can play Team Fortress 2 for free. That includes the Gold Rush update. You can pre-load the game now.