I'm sort of shocked how much I've been paying for web services these days -- usually it's always something that replaces software that use to be desktop based like QuickBooks. It also has to be of a high quality -- that or a situation where they're holding my data hostage and it's hard to export (Flickr is a very good example of this as I paid for the premium membership and now lacking time and a fear of loss I can't seem to stop giving them $$$).
API to make receiving stuff via emails (user sends email to create a blogpost/...) very, very easy. You cleanup incoming data, handle attachments, block spam etc. And cheap.