IFTTT (If This Then That) lets you automate tasks between popular web services such as Dropbox, Twitter, Evernote, Facebook, Instapaper, and many others. IFTTT works by telling it what task you want carried out, what will trigger this task, and it will put it into action for you.
A common example would be to have your Instagram pictures automatically backed up on your Dropbox or Box account. Certainly useful, but there's also more specific things you can do with it. For example, say I want to keep up with how a certain country is doing in the London 2012 Olympics.