I like my Burley raincoat:

http://www.burley.com/products/raing...int+Jacket&i=1

I'm not clear on all the ins and outs of variously breathable fabrics, but it has zippers flaps and other openings that manage to keep a *lot* of air flowing through while retaining enough coverage to keep me about as dry as I would be on a non-rainy day.

The black sleeves are nice for camouflaging grease stains that accompany the inevitable roadside rainy day maintenance...