geographical coordinate data sets for mashups?
pedalpete: Maybe you could clarify what you mean with an example. 
Are you looking for data that already is geocoded? if so, what type of data are you looking for? 
Or are you looking for ways to geo code data that you already have?I think looking for data that may or may not exist is the wrong way to go about your mash-up. Hopefully you already know what you want to mash.