There are lots of good choices, but for several years I've been using a Marmot Arroyo ($259; www.marmot.com) and think it as close to perfect for a summer bag. Rating is 30 degrees, and that's pretty conservative (I've slept in it at the 10,000-foot level of Mount Rainier). It uses very high-quality 800-fill down. The shell is a super-light nylon that is treated with a Nikwax compound for water repellency. Weight is very reasonable at one pound, 14 ounces. All in all, a fantastic bag.
Other options include Western Mountaineering's Ultralite, which is a touch lighter, yet also a little warmer with a 25-degree rating. Price is $290 (www.westernmountaineering.com). Feathered Friends' Road Runner is rated to 30 and sells for $235 (www.featheredfriends.com). Both these makers are down specialists and make some of the best bags on the planet. Marmot is a more broadly based maker of gear, but has always been highly regarded for the quality of its down products.
Of course, all those bags are also pretty expensive. Take a look at REI's more reasonably priced, new Sub Kilo ($209; www.rei.com). It's a 20-degree bag that weighs just a touch over two pounds, which is most impressive. High-quality 700-fill down provides the loft. The extra warmth makes it very versatile, usable for fall and spring camping as well as in the summer.