Yakima Skybox 18 Carbonite
I drive a double cab Toyota Tacoma with a camper shell so there's lots of space. But my wife and I also have a dog, a kid, a jogging stroller, and a ton of our own gear, so you'd be surprised just how fast the back get's cluttered up when we head out for a weekend trip.
To try and organize the chaos and make sure I can see out the rearview mirror, I've been using Yakima's Skybox 18 Carbonite cargo box. In the winter it works great for carrying and organizing skis: It will hold anything up to 215 centimeters long (remember those days?) and easily caries four or five pairs with tons of leftover room for boots and poles. In the summer, it will hold your camping gear plus plenty of beer.
Unlike some older roof racks, which require an annoying amount of assembly, the Skybox 18 Carbonite comes out of the box ready to go thanks to four simple pre-installed mounting clamps. Adjust the sliding clamps so they're lined up with your roof bars and then lock them around the bars by flipping latches inside the box; 10 minutes and you're done. The pre-installed clamps fit Yakima's round bars of course, but are compatible with square, aerodynamic, and factory bars as well.
Totally empty, the box weighs 52 pounds. It's a little cumbersome to handle, but I'm able to get it up on top of my truck by myself no problem. In terms of design, the box is built to reduce drag and I've never heard any crazy wind noise while cruising on the freeway. You'll see a reduction in gas mileage, but nothing drastic.
The box conveniently opens from both sides and Yakima designed it so you can't lock the key inside (it won't shut and lock without the key). The durable plastic should hold up to years of freezing temps, brutal sun, or anyone trying to mess with your gear.
If you need more room for some reason, Yakima also offers a gargantuan 21-cubic-foot version. Eighteen is plenty for us right now, but we'll probably have a second kid, so you never know.