iOS: Bop.fm is a great way to easily share music across services on the web. Its new iOS app makes listening on the go a heck of a lot easier too.
With Bop.fm, you can search for a song and provided it's on YouTube, Spotify (you'll need a Premium account to use Spotify) or Soundcloud, it will come up. You can then add that song to a playlist or play it right there in the app. Essentially, if you're using Spotify, it helps fill in the missing gaps in their library using Soundcloud and YouTube.
You won't know the difference or really care about where the tracks are coming from either, they will just pop up in search results and you can play them. There's also a social component and playlist sharing in the app, but it's far less interesting than the basic library aggregation.
Bop.fm (Free) [iTunes App Store]