HL2 didn't had a normal cycle of development, since a lot of parts of the project were developed at the same time - engine, locations, gameplay. In fact, it seems it was quite chaotic, to the point where significant parts of gameplay were changed when programmers added new features to the engine.
Thus, figuring what is Alpha and what is Beta is nigh impossible. Don't forget that majority of ideas from early days of development were trashed or significantly changed, so we are talking less about Beta, but more about "early development".
Arguably the closest thing to a traditional Beta are maps that were in the Leak: unfinished versions of maps from a final game. Everything else - are assorted concepts.
If anything HL2 Alpha would be fabled Hyperborea maps from 1999, when the game just entered actual development.
Кстати, раздел не тот.