Winamp

To create an .m3u/.m3u8 file in Winamp go to the Playlist window
and click List Opts->Save Playlist and then simply save the file.

iTunes

In iTunes, you first want to do an "Export Song List..."
You can right-click on "Music"...




...or you can right-click on a playlist to get this menu option.




Then be sure to save as a .txt file and not as an .xml file.