Nudie denim (non-selvedge) is pretty lightweight (fos says 11 ounces, but I call bullshit - I would say closer to 10.5 ounces)). And durable and relatively cheap (about $155-$180) as well. Rag and Bone has 11 ounce selvedge models (RB6s, for example), which are considerably more expensive, but also quite comfortable.
Rogan has a really lightweight denim. Crate is fairly lightweight as well. And last time I was at Barney's in Boston I ran across some pretty cool (well, maybe with the exception of the zipper fly) Engineered Garments denim in light summer weight, almost as light as the much more expensive Rogans.