Silk comforters are ideal alternatives to down. They are filled with silk floss and enclosed in a silk or cotton cover. They provide insulation, are naturally hypoallergenic, and lightweight.
SilkComforter.info is a great guide for anyone wanting to know more...
We've been thinking of getting one to replace our down comforter, as it is old and has started leaving feathers all over the apartment. The layered silk fibers of a silk comforter kind of adhere together and don't leak or bunch up. For cleaning, silk comforters can be hand-washed and tumble dried.
To purchase, these two sites look like a good start: White Loft Silk, Blue Stone Home.