If I'm just commuting to work I wear regular underwear under a pair of pants or knickers (sans any padding). Like Machka said, cotton kills (the skin at least) and so breathible frabrics are best. I like wool. For tours I used to wear regular shorts over padded wool (Ibex) cycling shorts but recently decieded that this was too warm for me and that I no longer cared if someone saw me and was easily frightened by the sight of me in stretchy cycling shorts. So I just wear my Ibex woolies (in winter and summer). That said, I do bring a couple of pair of undies on my tours for comfort wear, when I'm off the bike. I just wash them by hand when needed which isn't often since I don't get to wear them too much