Attention! Spoilers below!
So, if you've watched the Inception movie and if you're still trying to decipher the ending, you should have already noticed that you are not the only one in the world. There are a lot of discussions on the web about what exactly happened and there is a variety of possible endings.
While I think that the movie was entertaining to watch, there were still some issues with it. I do not intend to discuss them here except for the ending.
In my opinion an ending leading to a couple of possible interpretations is often considered original and interesting. The problem I see with the Inception's ending is that the possible interpretations are quite too many. There are also a couple of Inception's ending treatments coming to my mind that I haven't come across anywhere else that's why I'll try to list all of my interpretations here. And I think there are not substantial proofs in the movie sufficient to turn down any of those possible endings.
So, here is the list of all Inception endings coming to my mind (not ordered by any particular sign):
1. Cobb is still dreaming and he is in the limbo. Skilled Ariadne has designed a happy world for him and he continues his dreaming however he is not aware of this. I don't think this interpretation is quite probable but still, it is not impossible.
2. Cobb is still dreaming and he is in the limbo. Saito is also in the limbo that's why Cobb decided to stay there and have a "happy" life with his children and probably with his wife too because if Saito stays in the limbo, Cobb could not reunite with his children in the real life. Cobb behaves like a dreaming man in the ending so it could be interpreted he is aware that what is happening is not real. I don't think this interpretation is quite probable either but again it is not impossible.
3. Cobb is still dreaming but on level 0. This should be the reality level but if we look at the spinning totem we could assume that the reality is also a dream. In this case either Cobb himself or somebody else is constructing this happy dream.
4. Cobb is still dreaming but on level 1. For some reason he couldn't save Saito that's why it is impossible to be happy in the real world. Thus, Ariadne creates a dream world for him on level 1 - not very probable but not impossible also.
5. Cobb is in the real world. Everything described in the movie has actually happen and Saito has made the phone call to secure the happy ending.
6. Cobb is in the real world. There is not any dream technology. Cobb does not know any of the other passengers on the plane and he has just been dreaming while flying on the plane. There is not any device related dreaming but rather just an ordinary crazy dream. He has never lost his children. It's possible that he has also never lost his wife and just having a nightmare dream. In fact, this probability is very hard to be ignored - the whole ending starting with Cobb's awakening on the plane does not offer a single clue to disprove this version. (Click on the link to read more about this Inception Ending Interpretation.)
If someone could offer other Inception ending interpretations or could point facts from the movie that make some of the listed possibilities infeasible, please feel free to leave a comment on the topic.