Montane or OMM spring to mind, when ultra lightweight waterproofs come to mind.
My last purchase in the waterproof dept, was a Highlander cheap and chearful £19 affair, bought in the walking shop in Perth.
There's nothing technical about it at all, but blimey its waterproof, as i tried it up Glyder Fawr some weeks ago, in some heavy rain showers.
It may have only cost £19, but its fit for its purpose of keeping you dry in a heavy rain shower lasting several minutes.
Another good option, is to look at some of the very high tec cycling waterproofs, designed to keep cyclists drywhen the weather is rough.
Some of them are extremely light weight, and packable, but like most of the top end equipment aimed at cyclists, their not cheap, but their very lightweight, if thats your main criteria for buying one.
The Highlander is probably not very effective in heavy prolonged rain, but for a waterproof that comes in its own stuff sac, its a great addition to keep in the boot well of your motor, in case of an emergency.