I have a pair of water resistant tights for the light rain/misting/just cold days, which cut out most of the front drenching. They're a previous version of these, I think: http://www.teamestrogen.com/prodSU_40552F.html

For serious downpour, a cheap pair of waterproof shell pants for skiers, used with pants clips. They make a ton of noise (zip!zip!zip!zip!), but NOTHING gets through those.

-- gnat!