This probably would have been better to release before The Avengers came out, but better late than never: Marvel has created this nice infographic that lays out the official timeline for the Marvel Cinematic Universe. It covers every movie before The Avengers, as well as additional events either from minor spin-off material or simply inferred along the way. There are no hard dates, with everything listed as happening either before or after Tony Stark's announcement that he is Iron Man at the end of the first film. I'm not sure why that event is so important, but I guess they really don't want to tie themselves down with a specific time period.
If you've seen all the previous movies, there's nothing too unexpected in here. The only surprising thing is how close together some of the events are. You'll notice a section of the timeline called "Fury's Big Week," which details how The Incredible Hulk, Iron Man 2, and Thor all pretty much happened at the same time. I know what they're trying to imply that Tony Stark sort of "opened the floodgates" and was just the first sign of all this unusual activity. But while Iron Man, the Hulk, and Captain America are all linked by Howard Stark and his work on the super soldier serum, what are the odds that S.H.I.E.L.D. has to deal with the events of The Incredible Hulk and Iron Man 2 at the same time they make first contact with extraterrestrial life in Thor? It's odd to say this for comic book movies, but it's a bit farfetched.
Sadly, you can't use this to skip over watching the movies you're not interested in. I'd love to give this to my mom, who only watched the Iron Man films because of Robert Downey, Jr., and tell her to go see The Avengers. However, you still need some knowledge of either the movies or comics to make out what the events in the timeline mean, so it's no easy shortcut.