Lots of info about various layering options on
Mark Verber's website. He gives a good overview ranging from the latest innovations to low budget options.
The ideal rain shell would be one made of eVENT or an equally breathable fabric, with pit zips and full zipper for more ventilation (even the most breathable fabric gets sweaty inside with exercise), weighing no more than 4 oz. and, of course, costing under $100. Since a jacket like this is definitely in the "unobtanium" class right now, most of us take the lightest we can get and can afford!
I personally haven't found Goretex any more breathable than plain old coated nylon, and my budget doesn't run to eVENT at this point. I therefore stick with the coated nylon (actually, silicone-impregnated nylon) when it's cold and just get wet when it's warm (making sure I put on an insulating layer plus rain jacket when I stop). Your Mileage May (and probably will) Vary. I just bought a Marmot Precip jacket for around home, but I haven't yet had it out in prolonged rain. I already found that I have the usual problem of getting too sweaty inside even with the pit zips open.
Unless you're going to be doing brush bashing or rock climbing, DriDucks is a highly breathable, lightweight, inexpensive solution for which many people opt. It isn't durable enough for anything off-trail, though. The front zipper stitching may need to be seam-sealed (test it in the shower).
You need to be sure you get something that is WaterProof and Breathable (WPB) rather than Durable Water Repellent (DWR). The latter may work for a light drizzle but definitely will not keep off prolonged rain.