If it's warm, say above 70F I just get wet. Below that I wear O2: http://www.rainshield.com/. It's very light, cheap and breathable but very delicate, easy to rip. I got the 2X size so I can stuff a sweater or fleece top underneath and still feel kind of light. For cold/wet (lower than 40F) I use a cheap, rubberized Columbia jacket I got on sale for like $30. It's not breathable, just has vents underarms and under the flap on the back. I can't find a link for that, I think they don't make that anymore.

And I use Axiom Monsoon and Typhoon waterproof panniers.

Adam