I am an admitted case ***** and tend to cycle through them regularly depending on what I feel like looking at that particular day. These are some of my favorites:
Speck CandyShell (speckproducts.com) - has a hard plastic outer shell and a soft rubberized inner shell, so it both protects against scratches and impacts from falls. It comes in a bunch of colors and if you use Twitter, sign up to follow them, as they often tweet discount codes, including fun "theme" ones (e.g., for Valentine's Day, they gave out a code that gave you 25% off any red, pink, or purple case). This case is one of the bulkier ones out there (though not obnoxious by any means) but provides outstanding protection, especially if you're prone to the occasional drop.
(these are the new colors...IMO, the earlier ones were nicer, so check the site to see all options available)
Incipio NGP (Next Generation Polymer) - is a so-called "soft plastic" case. Probably my favorite so far; I'm using a knockoff NGP right now only because Incipio didn't offer the color I was looking for (bright, sunshiny golden yellow). I've been switching back and forth between it and the pink NGP for weeks now.
iSkin Solo FX - similar to the NGP; it's just available in a different style and brighter colors. There are TONS of knockoffs available on ebay/
Amazon, but be careful which ones you buy if you decide to give them a try. Some obviously didn't use a lot of care with regard to button coverings, cutouts, etc. I have 2 knockoff cases that are totally unusable because one presses constantly on the sleep/wake button, and the other does the same on the volume rocker. EXTREMELY annoying.
I know a lot of people like the InCase Slider, but after hearing horror stories about deep scratches caused by this case on the backs of people's phones, I've stayed the heck away from it.
I've also tried and liked the iFrogz Luxe and the Incipio Feather, but keep coming back to the NGP/iSkin-type cases. I just dig 'em.
Oh, and a site that I've found that's good to deal with is WirelessRage.com. They have pretty much every style imaginable, and also tweet out discount codes and announce when they get new styles in.
OK, apparently the filter didn't like my self-description, so I amend it to say that I am a case "lady of the night."