http://www.polarwrap.com/default.asp...1&sid=37&show=

I use these and my winter riding buddies estimate that you can ride with comfort about 10 degrees Fahrenheit lower that you would be able to without them.

They do add some bulk so you will have to make certain that it does not make your shoes too tight and restrict blood flow thus making your feet colder. I have not used the thinnest version.