I'm a big fan of OtterBox... I have a waterproof case (Armor series) for my 1st gen iPhone, plus ruggedized cases (Defender series) for that 1st gen and 2 more (one white, one pink with proceeds to fight breast cancer) for the 3G I just bought at BestBuy on Saturday.
But they recently introduced smaller rubber/silicon cases (Impact series) like you're probably looking for, for the 3G. I have one of those, too (Full disclosure, I write for iPhone Life magazine, so they gave me a ton of stuff at the Consumer Electronics Show) But despite the freebies, they truly are the most rugged cases out there. Even the new slim case has air pockets on all 4 corners to protect against drops as that's the likely point of impact. Check it out at
www.OtterBox.com. I also have a couple of photos up at
www.PlanetOtterBox.com (Rock of Gibraltar and Island of Atlantis, Bahamas).
I have Speck cases too (again, full disclosure, they also gave me a bunch of freebies) and they are quite stylish. I've only had my 3G for a couple of days, so I haven't settled on any one yet. But if you are afraid of drops, OtterBox is worth considering.