Rain Gear is always a bit of a problem, because you're always in a trade off between breathability on the one hand and waterproofness on the other. It's possible to stay dry in some truly horrendous conditions, but you'll probably end up sweating yourself wet, and likewise a highly breathable rain system is going to get you wet from the outside in. The manufacturers below have not solved this dilemma, but they've at least attempted it in a lightweight way, which is something at least.
One other note, is that those looking to trim every ounce of weight might want to look in to a tarp / poncho system. This is where your rain gear consists of a poncho which converts in to a tarp. In the summer for example, or on a trip where you're reasonably sure you're not going to be the victim of any particularly bad rain storms, this is a great way to save weight, which is why it's one of the most popular dual use pieces of gear.
For reviews of various rain gear, see here.
GoLite is a well established lightweight company. Though some of their products do drift towards the heavier side of things, they make a lot of excellent gear.
Integral Designs is a manufacturer that uses mainly eVent fabric for their waterproof products. eVent is similar to Gore-Tex, though typically lighter and cheaper. They also carry a few poncho / tarp combinations.
Mountain Laurel Designs
Mountain Laurel Designs don't make much as far as clothing goes, but what they do make is fairly innovative. They have rain chaps, eVent mitts, and a cuben fiber poncho / tarp combination, among other things.