Hopper does this with our "Flex dates" feature. You can specify a date (up to a year out I think) and stay (anywhere from 4 days to 2 weeks) window for when you want to fly as well as regions you're interested in traveling to, instead of just 1 destination (although we also allow you to select just 1 destination).

It was sort of hidden for a while because we were testing other features, but it's back on the main search screen now.

Disclaimer: I work for Hopper

Edit: Forgot to mention this feature is only available on iOS at the moment...