You could try Frogg Toggs, Light, cheap and breathable. You don't get much durability though, but it shouldn't matter too much to cycling. You can get a suit for less than $20, and it breathes better than any fancy rain shell I've used. The fit is terrible though, and I'm a little embarrassed to be seen in it off a bike.

You could also use a wind shell with decent DWR over a wool base layer, which works pretty well for light rain. Breathes much better than any real rain gear,and while you do get wet, you stay comfortable and relatively cool.