It's $0.99, but it's pretty slick: PlayList Builder - Android app on AppBrain
You do need to generate new playlists if you've added new music, I believe (but someone who cares should check), but even that would be pretty easy.
How it works is that there are a bunch of fields that you can populate, and then when you've entered all the filter criteria you want, you can export a playlist that will automatically have all the songs that meet the filter critieria.
Of course you can do this on a computer for free, too... But I've seen people requesting the ability to easily create playlists while away from a computer.