I have several pair of Campmor branded 100 wt fleece pants that I got for dirt cheap. Very comfortable. North Face sells them – TKA100 fleece. Columbia also sells them under their Fast Trek model:
http://www.campmor.com/outdoor/gear/Product___59798
I don't wear them much anymore. I've replaced them with PowerStretch tights and tops. Pretty much the same thickness as 100 weight fleece, but in a stretchy fabric that fits tight and has a dense woven exterior (fleece on the inside).
These are awesome pajamas in very cold conditions. Can be used as expedition weight baselayers. The pants work great under rain shells for snowy active conditions (like snow shovelling). The tops are an easy-care alternative to sweaters all winter long.
Quite a few companies sell the. EMS, Marmot, Patagonia (Piton), etc.
Also, the new Polartec Power Dry High Efficiency fabric is fantastic for this kind of use and a bit lighter in weight than PowerStretch. Patagonia makes Capilene 4 tops and bottoms in this fabric.
