The open cardi that I like are these lots from All Saints, by which their knitwear is always my all time favorite due to them giving something different compared to the basic look Uniqlo or H&M used to have. Most of them drop way below your waist to give the cardi-meet-jacket kind look, which is pretty helpful if I wanna opt not to wear a usual blazer for that day. Material wise ranges from fully cotton to linen/cotton that I think is quite lightweight; and cotton/cashmere to wool and alpaca/acrylic that are quite heavy and much 'warming'.
I like the designs: from the simplicity of Alexi and Flex, to the penguin-like tail of Brooklyn and Chicago to the frill front of Edge and Morris which resembles what Songzio did for Spring/Summer 2011 (or also look like most of Valentino dresses!).
Some of them got an oversized pin to tuck both parts together. Or if you're a daring one, wear it with a USB wrapped around the waist. USB as in the ultra skinny belt, not the pen drive, fyi. :)
All Saints Flex hooded cardigan
[pix from all saints]