I first came across Xtranormal on the Chronicle of Higher Education forums where people were making videos recreating favourite (that is humorous) conservations with students. Xtranormal enables you to create films using stock actors and sets, but typing in a script for the dialogue. The actors are comical looking and 'robotic voices' which encourages its use for humorous purposes. However, I have been using it as part of Getting the Most out of Feedback (GMOOF) project which is a sub-project of the Sharing Practice in Assuring and Enhancing Quality (SPEAQ) project.
The free service offers a fairly limited range of stock characters and scenes. This is good to get started with, but a subscription unlocks a lot more characters and functionality. Xtranormal videos can be made online, but I would recommend downloading the desktop versions which I found was faster and enables the user to have up to 12 characters in their movie. (I understand this is only available for Windows at present). The videos are saved in .slate format. These can then be exported to other formats which enable the videos to be viewed in video player software or uploaded to the web.
I have uploaded the videos I made from the feedback project to youtube. The videos will form part of the resources on the GMOOF website. One of the videos in the GMOOF Youtube list is below.