In my day job, we're often asked to pull reports in Excel directly from various APIs that we provide. I thought it would be cool (and perhaps generally useful) to do that straight from a Web API call with minimal setup, so I decided to work on the idea at home.
So far, I have a fairly limited MediaTypeFormatter implementation that uses EPPlus to serialise an IEnumerable of a class to a formatted Excel spreadsheet. The options are fairly limited (e.g. only IEnumerable<class>) and I need to improve documentation and unit test coverage, but it's functional and under active development. Cell and header formatting can be specified on the formatter instance, and there are custom attributes to allow the generated file name and headers to use when serialising a particular class.
Is this something you might be interested in adding to WebApiContrib when it's more complete and better tested? I know the project isn't a dumping ground for every vaguely-interesting thing related to Web API, so I won't be devastated if you don't think it would fit or it's a little too niche! :)
(By the way—wow, fast work closing that issue! I think I blinked and it was resolved already?!)