The Heroku Twitter-bot Starter Kit
Get yourself a Twitter bot up and running in no time!
Just run the init.sh script, follow the instructions, and you'll be good to go.
Note: While this was once a free method for running a Twitter bot, Heroku's billing has changed and it is no longer free. One could certainly run the bot on an AWS micro instance (free with the AWS free tier), but there's no automation built for that.