I have Patagonia's Capilene® long johns.
I second Patagonia Capilene. Their expedition weight offers a lot of bang for the bulk. For max effect, layered with a pair of lightweight or silkweight next to the skin, with a pair of expedition weight over that and then your fleece and then your shell.
Oops. It looks like they've renamed their products, It looks like 'silkweight' is now called 'Capilene 1' and 'expedition weight' is now called 'Capilene 4'.
-john