[davmre]

You can actually do this with gmail itself: [e-mail] is a valid alias for [e-mail], for any value of x. So you can make up whatever set of custom addresses you want; they all go to the same account, and filtering/forwarding/etc. can be done with standard gmail filters (not sure about autoresponses; you'd probably need a third-party client for that).

Of course this plus-sign behavior is a pretty well-known fact about Gmail, so it'll be obvious that you're bucketing your email in this way. Anyone can easily read off the 'base' address [e-mail], so you'd need to set it up so that mail to this address is heavily restricted and anyone wanting to actually reach you would have to use [e-mail] or whatever. But I think this would basically also be true of any custom service you could invent having similar functionality.