Note: This site appeared to be no longer active as of Summer 2016.Check out Simitator instead.
Fake Tweet Builderhttp://faketweetbuilder.com/
Give your students a fun and creative chance to create fake Twitter conversations by historical figures, book characters, politicians, authors…
The interface is incredibly easy to use, and allows you to add custom avatars, background colors and images, and more. Your fake Twitter thread can downloaded as an image file.
Need some examples, inspiration, or just a good laugh? Check out http://historicaltweets.com/. I would copy and paste examples from this site rather than share it directly with students though as some of the content might not be appropriate.