The only hotel chain to do this directly is Hilton. As indicated above, you can do others with Points.com, but the exchange rate isn't the greatest.

IMO, this is a good idea if you need a few points to top off your Hilton account for an award. If you have so many miles that you can't figure out how to use them all, and no prospects for acquiring a more active imagination or no time for more trips, then you also might want to exchange miles for points.

I have no problem using my miles, so I find that the least cost idea is to use the miles for award airline tickets, and to use Priceline for hotel rooms.