The web app is located at http://videodropper.ep.io. Do you have any suggestions for ad networks that's suitable for small non-content based sites?
However, I'd also suggest looking into alternative sources of funding, or alternatively just treating it as a hobby. Also note that numerous video downloaders already exist; "send to Dropbox" does not necessarily make yours stand out, and it will cost you quite a bit of bandwidth.
You might also look into hosting which provides unmetered bandwidth, such as gandi.net.
In terms of monitization I think you're screwed, unless you got down the banner ads route which is going to make the site look terrible.
In any case, I would recommend influads as well. They are currently working with quite a few people with the same situation as you - people that have an app with moderate success and are looking to incorporate some non-intrusive, good-looking, and relevant advertisements.