You could try Paramo over trousers. They have full length zips which aid venting. They're waterproof and very breathable. You can get them in places like Field & Trek or other outdoor shops. Pricey though, also good for hill walking etc. I wear them as trousers rather than over trousers. Another good feature is that they can be repeatedly reproofed so last for ages. Nice material too.
I bought a pair of Endura Thermastat Bib Longs for winter last year.
they are great apart from the fact that they were too long, but then I am a bit of a stocky short-arse, so anyone who conforms to the XC stereotype would be fine.
They are very warm, even when they get wet, and I used them through the depths of a Scottish winter
Eager Sport make overtrousers with mesh backs to the legs and no arse, i.e. you sit on your tights or whatever. They are tapered and tight around the legs, long zips, and you don't sweat. You can zip them over your overshoes. You only need waterproofing on the front when riding, so I'd say they were a good solution. Just don't walk around in them, you'll look a little pervy. Cheap too (£40 or so). John
i use altura cascade waterproof trousers, they are ok, not as breathable as Gore but only £40 good value. ideal for commuting and long winter rides. for winter fun riding / jumping I use Fox Torque pants, full length moto-x pants which are warm and comfy.
I have the Altura trousers for commuting when I don't want to get my work strides wet, but for normal/proper riding I just use a raceguard to keep my arse dry and accept that my legs will get wet/muddy. For DHing when it's atrocious I have a waterproof motorbike all-in-one suit (from Hein Gerike in Cardiff, about 15 quid) to keep the worst of the crap off. But it's sweaty.
