I'm not sure which mod this is, but I've seen one that would make a chest pick up items that drop near it. Maybe would be abused for farming, but would make a great way for users to donate items. Maybe only mods could set the property for a user.
On a similar note, but not sure if there's anything like this, a block that could generate a signal when 'paid' a certain amount. This could be used to automate user markets, but also to automate 'mini-games' so the owner doesn't need to be online to run it.