Griffin: If you're looking for a really wide selection of iPhone case styles, Griffin offers a wide variety (although the iPhone 5 editions aren't yet available for ordering). If you're willing to wait, Griffin has anything from solid colored protective cases, to ones in animal shapes, to ultra-thin hard shell cases in fun colors.
Case Mate: Although not all cases are available yet on the Case Mate site, there are at least 12 styles with various color options that you can order now. They are carrying anything from their signature Barely There cases, to ones with animal designs, quilted material, exotic hardwoods, and even one with a stand embedded into the case. My personal favorite is the sparkly Glam case.
Incase: Incase is often fashionably late when it comes to outfitting the latest iOS product...but they're usually the first company we think of when we're looking for the most stylish options. If you're a fan of their classic Slider cases, you'll be able to outfit your iPhone 5 with one in black or white, or choose from their colorful Crystal or Metallic collections options. They also have a Snap case available, along with a Sports Armband for those who like to use their phone during workouts. Otterbox: Otterbox's Commuter Series are cases that have two layers of protection built in, and the additional protection of an adhesive screen protector. If you tend to drop your phone, these are a good choice to go with. They come in a variety of two-tone colors, or you can go all black for a simpler look.
Incipio: If you like silicon cases but want to feel at ease with something that provides a little more protection, Incipio's cases vary from semi rigid soft shell cases to ultra thin hardshell. I really like their LGND case, which is a hard shell folio, as well as their Frequency case in Cherry Blossom Pink. (Images: as credited above)