What's the simplest way to take a series of .mp3 or .wav files, and turn them into a shareable Spotify playlist?

[ARNK]

I used Spotify as my example in the question, but I'd also be interested in taking .mp3 or .wav to an iTunes playlist or a Windows Media Playlist, etc. Is there a program that allows for this? The point would be, once I have the playlist file or link, I can send it to a friend or acquaintance, and it'd simply be added to their playlist in whichever program they're using.

 

Thanks in advance!