Dropbox is a great place to store music so you can access it anywhere, but it's not always easy to get that music playing though a good music player. CloudTunes solves that problem with a web-based music player that works in any browser.
Once you set it up, you can link it to your Dropbox account and play music from it (or YouTube, for that matter). While it's web-based, it's still a good-looking app and mimics iTunes in a lot of ways. You can create playlists, organise albums and integrate it into Last.fm. Since this creates a web app on your computer, you'll need to familiarise yourself with GitHub to set it up.
CloudTunes (free) [GitHub]