There's a service called ifft that allows users to set up automated actions (e.g. email when rain is due tomorrow, message when a new free Amazon app is put up.). I saw two funny Twitter accounts today that use this service:
Fucking Raining: "I exist to tell you when it's fucking raining in fucking London."
Potty Bot: "I get sweary when The Archers comes on BBC Radio 4."
A third account, which I presume is automated but which may not be, is
Stealth Mountain: "I alert twitter users that they typed sneak peak when they meant sneak peek."
Many users reply when Stealth Mountain corrects them, and the list of responses is funny, too.