It depends on how heavy the rain is or is supposed to be. I often just wear my regular riding clothes and get wet. Like others have said, I prefer rain to sweat. I did recently buy a $3 Coleman vinyl poncho at Big Lots or a similar store and recently tried it out on a ride to work in some heavier rain. It worked well and kept me dry and relatively cool, since the sides just kind of button up.

Another vote for good fenders, too.