CloudTunes is an open source webapp that with a unified UI for music stored in the cloud (YouTube, DropBox, etc.) and integrates with Last.fm, Facebook, and Musicbrainz for metadata, discovery, and social experience. It's similar to Spotify, but instead of local tracks and the fixed catalog, CloudTunes uses your files and YouTube music videos.
iTunes Remote is simple web app to control a native MacOS app. With it you can control your iTunes app remotely without Internet!
A SoundCloud app which integrates into your desktop. Never do you have to switch and find your browser tab again. With these native desktop integrations, you'll be listening to SoundCloud while being more productive.