Media Monkey will do this. You can click on a folder in the little side file manager, and in when it opens the sub folder list you can either click on a sub folder or it has "all" listed. It will then open all the media files in the center window, and if you want to play everything in that folder just right-click "play now" and it loads the whole folder into the playlist.
Amarok works similarly, but you have to select all the files, then right-click and either select "add to playlist" or "replace playlist"--works fine, but not as convenient.
If for some reason you need actual playlist files, you'd have to create the playlist file after you loaded the files. But loading a whole folder and playing is incredibly easy with Media Monkey.