Man, could you only imagine how HL2 would have turned out if the massive beta leak didn't happen. Valve did an excellent job on the released HL2, but some of the enemies and areas that were in the beta and concept art would have been awesome.
A leak???
In 2003, someone hacked the Valve servers and took map files of HL2, HL1: Source, CS:S, and the code for the Source engine and published it over the net. HL2 was suppose to be released in September of 03 but the leak caused it to be delayed a whole year, which I have a feeling is where Steam was developed and where many changes were implemented into the game that we have today.