The cookbook Super Natural Cooking by Heidi Swanson has lots to say about how to sweeten things. She doesn't like ordinary sugar much, as I recall, but has all kinds of other cool suggestions for less processed sweetening. The library is my favorite source for cookbooks.
Also, at least one of the books I've read recently touted nutritional benefits for unfiltered honey and blackstrap molasses -- I use a mix of those in my bread-baking most of the time. I suspect it would also work in muffins that you didn't want to be particularly sweet tasting.
I haven't used any refined sugar since I started South Beach Diet in May. I use agave nectar which is a low glycemic sweetener. Websites help people learn how to substitute it in baking. Here's a link: http://www.allaboutagave.com/