I'm a big fan of these: Fjällräven G-1000 trousers
A blend of 35% cotton and 65% polyester.

Breathes really well, very comfortable (even when wet) and dries really fast. If it only rains a little I won't bother, but when it pours rain or really wet snow I'll augment with waterproof trousers (gore-tex equivalent).

For the record I also used this heavy-duty Norrönna canvas jacket for many years. 100% cotton in a very thightly woven material. This works really well in sub-zero temperatures. It works OK in sleet. In rain it's not optimal, but it won't kill you either: It takes quite a beating before you get soaking wet, and it will keep the wind out even when wet. But there is a reason I didn't buy another one, but replaced it with goretex. http://www.norrona.com/Products/0118-07/svalbard-arktis-cotton-ano-jacket-mw
The properties of these canvas-like materials have very little in common with your average daily wear cotton material, such as jeans. Jeans are close to absolutely lethal in anything but extremely dry outdoor settings.

EDIT: Recalling fond memories about the trips done with this jacket I also remember the marked shift in water repellent properties that occurs around 8-10 Celsius. Below this temperature it sort of works, it'll keep you adequately semi-dry for quite a while. Wear wool underneath and you're OK. Wear it in the rain above 10C and suddenly I was instantly soaking wet... As I said, there's a reason it is a nostalgic memory replaced with gore-tex equivalent.