Half-Life 2 Episode 1 is a continuation of the main Half-Life 2 game. It comes in the "Orange Box" collection that retails for $50. This is a fun, short game set in the Half-Life 2 universe.
You continue to play as Gordon Freeman in this game. With your trusty crowbar, gravity gun, and all sorts of kinetic weapons, you set out to save the day again. The gameplay is similar to the HL2 game, where you shoot things and use the gravity gun to solve easy puzzles along the way. Don't expect to do anything new since you'll be sorely disappointed, but the game has a fun storyline that advances the adventures of Gordon Freeman against the Combine threat.
This game is an exciting addition if you enjoyed the original Half-Life 2.