Dfilm (now known as Dvolver)
Dvovler is a great website which lets students make their own short movies. They can choose the characters, the setting, the music and write the dialogue. When they've finished, they can email the films to their friends.
I usually get them to choose a genre, (romance, sci-fi and so on) and write a script for two characters, just six lines long. Make sure that they don't write long speeches (as you can see below, there isn't room for more than 20 words(ish) in each speech bubble). They can add extra scenes if they want to make the film longer.
It's also a good idea to get them to write the script before they make the film on the computer - that way their imagination isn't only constrained by the settings and characters provided. Also, it means that if a computer crashes and they lose what they were doing, they have a paper version of the script.
I've used dvolver to practise using phrasal verbs, idioms, collocations and different grammar points. I'm sure there are other ways you could use it though.
For example, an "alphabet dialogue". The first speaker must start with the letter "A", the second with "B" and so on.
Are you waiting for someone?
Betty. She's my girlfriend. She's always late.
Couldn't you just leave?
Definitely not - she'd be really angry.
... And so on ...
Another movie making site is xtra normal. It's a lot more complex, but the characters actually speak your script and the animation is 3-D. I haven't used it with my students yet as you have to create an account and I find that all too complicated.