[THEORY] All the Valve games are in THE SAME UNIVERSE!
I have a theory that all the Valve games are actually in the same universe. I have facts to support it too! Here is how the timeline would go:
First comes Team Fortress Classic. The TF2 comic, "A Cold Day in Hell" shows that Team Fortress Classic is cannon. The characters are still alive too. My guess is that TFC would take place 5 or 10 years before Team Fortress 2. This would set it around 1930's or 40's.
Next comes Team Fortress 2. Calenders in the game, and dates in media and comics, state that it takes place around 1950's or 1960's.
The events in Portal, Half life, Opposing Fource, and Blue Shift, and Counter Strike probablly take place around the same time. We know that the Black Mesa incident takes place around 5 years before or after Portal. Portal takes place around 1999 or 2000, so the first Half Life games would be around late 1980s or early 2000's. Based on the locations and technology of Counter Strike, it would take place around 1980 to 2000's.
Next comes Half Life 2 and it's episodes. Valve stated that Half Life 2 takes place 2 decades after the first Half Life, so that would set Half Life 2 around the 2020's
Portal 2 would take place after the Combine has been defeated from it's takeover of Earth, because in the ending of Portal 2, when you portal onto the moon and you can see Earth, the oceans are not drained of water, and there seems to be no Combine technology. Based off of the amount of deterioration to the Enrichment Center, and the expiration dates on the Longterm Relaxation Vaults, Portal 2 would take place 20 or 30 years after Half Life 2, so it would be 2040 or 2050.
Now for the final pice of the puzzle: The Left 4 Dead series. In the games, a zombie apocalypse has occured. There is no evidense of a "zombie apoclypse" in the Portal or Half Life games, and the game could not take place before Portal, because of the time period. So this sets Left 4 Dead sometime during or possibly after Portal 2. The zombies could even be a mutation of the headcrab zombies from the Half Life games. The zombies could also just be an infection or a virus.
I hope you enjoyed my theory. I think it makes sense, but it really depends on the opinion. Thanks for reading!
UPDATE: Thanks everyone for commenting and sharing your opinions! When I first posted this I thought I was crazy. . . Yes, I realized that the Counter Strike timeline may have been confusing. I meant to say that the CS games probablly took place before the Combine takeover of earth, so that would put it around the events of portal or the first Half Life games. Secondly, I know I forgot about Dota 2. I haven't really played the game and I honestlly never even researched it when I made the guide. My guess is that it may take place on a different planet or EXTREMELY far into the future on earth.