Windows only: M3U Dropper is a lightweight portable program that creates music playlists using songs from your music library or from online links without having to open up your media player.
Through file drag-and-drop, M3U Droppers adds songs stored on the computer without the need to open up a music player. (Through the preferences, though, a default music player can be set to play songs dropped into the program.) Likewise, by dragging a link from a browser into the program, M3U Dropper automatically adds an online song into the playlist.
What's nice about the program is that nearly all of the functions to organise songs can be done via hotkeys. My biggest quibble about the program is that other than dragging and dropping, there's no other way to add songs to M3U Dropper — but better that than the other way around. M3U Dropper is a free program for Windows only.