Quite a few years ago, before Half-Life 2 launched, someone hacked into Valve's systems and stole the source code for Half-Life 2 and leaked a copy online. Gabe took to the Internet and asked for help in tracking down those responsible and later with the help of the FBI the hacker was caught. I recently watched a video that shows off the Half-Life 2 beta and it's a complete run through of the game at that point from start to finish. As you can see the story is a bit different as is the gameplay and art. It's interesting to see Half-Life 2 in such an early form compared to what it was when it launched(the final version being much better in my opinion).

http://half-life.wikia.com/wiki/Half-Life_2_Beta