Here are the places I've gone for herbs:
The low-level herbs can be found anywhere, so I won't bother.
For the Alliance mid-level alchemist, Wetlands. All the herbs you need can be found here-- Steelbloom in the mountains, Stranglekelp off the coast, Liferoot in the swamps, and the invaluable Kingsblood in many places in between.
For the 30s and low 40s, Stranglethorn Vale, Badlands, and Swamp of Sorrows. STV has lots of Goldthorn, which is a big reason to go there; also more Steelbloom, good amounts of Khadgar's Whisker, occasional Kingsblood, but the *real* attraction is Fadeleaf. Rogues need copious amounts of this stuff, and there's a fair amount of it in STV. On a PvP server rogues will pay out the nose for Fadeleaf; sell it in stacks of ten to maximize profit. You can also get Purple Lotus in STV if you're willing to chop through the trolls.
Swamp of Sorrows gives you more Goldthorn and Liferoot and absurd amounts of Blindweed, which powers some very nice caster potions. In Badlands you can find considerable Firebloom, and lesser amounts of Goldthorn, Whisker, and Kingsblood. Tanaris is another good place to go for Firebloom... especially since there are no other herbs to be found, apart from the occasional Purple Lotus.
If, for some reason, you need Wintersbite, I recommend Alterac mountains. Take the road North from Tarren Mill. Stay where there's snow. If you have a mount, you can scour the area pretty quickly. In addition to Wintersbite, you can also find Goldthorn and Whisker there.
To get Sungrass, you can go to Ferelas or Hinterlands. I've found Hinterlands to be preferable, but that's possibly due to my alchemist being Alliance-side. Hinterlands has lots of Sungrass, and also the occasional Golden Samsa. Follow Zxtole's advice if you need Ghost Mushrooms.
That's all I could think of for now...