Web-based music application MixTape.me lets you create, listen to, learn about, and share playlists quickly and easily. It's awesome.
Full Disclosure: I'm completely biased, of course. MixTape.me has been a pet project of mine over the past year or so—but something that's been brewing for the past several years.
The purpose of the site is simple: It's meant to look, feel, and act just like desktop music player—but it lives in your browser. Creating a playlist works the same way as it does on your desktop. Search for a song you want to add, drag and drop it to a playlist (or to your disposable Quick Playlist drop box), and enjoy. I could spend more time describing how it works, but the video demonstrates it better than I could ever explain it.
The site integrates with several other music services to integrate artist bios (Last.fm), lyrics (LyricWiki), and YouTube videos with the music you're listening to, allowing you to learn about the music as you enjoy it. Once you create a playlist, you can embed it anywhere (à la YouTube), like the Graduation playlist below I made in honour of MixTape.me's release into the wild.
There's a lot more to it than that, but I'll leave you with that, and with a note: MixTape.me is a pet project that is almost guaranteed to have some bugs. I've had no major problems with it in any major browser, though Internet Explorer is its least favourite. It's been most extensively tested on Firefox. Fingers crossed that the server can handle the Lifehacker traffic. I'll be banging my wrench against the server all night in hopes of keeping it up and running. Thanks in advance for trying it out!
If you would be so kind, please send any problems or bugs to support at mixtape.me.