The best thing about having a game with online capabilities is the fact that you can play with your friends. However, if your friends aren’t around (or you don’t have any) bots are the next best thing. Previously, Left 4 Dead 2 did not feature bots but Valve has continously said that they will bring them to the game soon enough. And do that they have with their latest update which also includes a few tweaks to the game’s SDK, which you probably don’t care about unless your a developer.
Additionally, Valve let us know that a similar patch for the Xbox 360 is in the works and should be ready soon.