I’ve tried a few online movie catalogues in the past and have always been disappointed with either the user interface, the export features or the accuracy. I somehow stumbled upon Ajaxilicious::Movies this evening and I must say that I’m fairly impressed with it as a whole.
The developers describe the service as:
Ajaxilicious::Movies is an online movie catalogue for managing your movies and sharing your lists with others via RSS or using our interface. No matter where you are you can access your movies and update your catalogue fast and easy with the help of AJAX.
They also have API documention available as well.