here's a couple that haven't been posted yet:
western mountaineering - Western Mountaineering
and feathered friends - Feathered Friends -- Welcome to Feathered Friends
both of these companies specialize in down garments, but they're not cheap.
i have a couple of westerns - the antelope in the smf. i think it's rated to 5 or something like that. the other one i don't remember the name (apache maybe), but it's a 15 deg bag and is the dryloft material. i've slept in the open in the 15 deg bag at 14 deg according to the thermometer, and i was toasty. the 15 deg bag packs down to about the size of a large football and weighs just over two pounds. the antelope is really warm, too warm really for me unless it's really cold out. i've been down to about 10 in it, and i still got a little too warm. packs up a little bigger than the other bag. their bags are rated at least 5 degrees warmer than they are good to, unless you're a really cold sleeper.
i had a feathered friends bag too. great bag. super light. quality construction. packed down super small. don't really know why i sold it, but i did, and i bought the 15 degree western. guess that's enough said.
whatever you buy, buy down. way warmer for the weight. and if you buy something that's too warm for the summer, just unzip and use it like a quilt. that way you can hang a leg out or kick it off partially if it gets too hot. that's the only way i sleep in a bag now unless it's really cold. much more comfortable too if you're the least bit claustrophobic.