CouchApps are JavaScript and HTML5 applications served directly from CouchDB. If you can fit your application into those constraints, then you get CouchDB's scalability and flexibility "for free" (and deploying your app is as simple as replicating it to the production server).
For a technical overview, read What the HTTP is CouchApp?
jquery.couch.app.js, and PathbinderFiles attached to Simple JavaScript Applications with CouchDB: