Windows/Mac (and Chrome/Firefox): Music discovery and sharing services are limited by their platforms. You can thumbs-up, "Scrobble" and otherwise feed songs to desktop players and YouTube, but those music channels can't see each other. Like.fm aims to track your listening everywhere, then connect you to other tunes you might dig.
Sign up for Like.fm (manually or through Facebook), and you're prompted to download its client for Windows or Mac, as well as an extension for your browser. These tools hook into just about everywhere you're likely to hear music: iTunes, Windows Media Player, YouTube, Grooveshark, Rdio and so on. The app tracks your tracks in near real time, and when you look through your history you can mark tracks to be added to a custom playlist. Head over to that playlist and click play, and your music streams through — often through miniature YouTube windows, which is a clever way of getting you your music. As you listen to more stuff and mark it as a favourite, Like.fm also gets good at recommending other music to try and users to follow.
Like.fm is a free service, and available for Windows and Mac music clients, as well as Chrome and Firefox.