This thread's a bit old, but a good starting point.
Also,
this article about DIY bitters is a good read.
Here's another good article about making your own bitters.
Finally, these two small-batch brands are excellent:
Bittercube out of Milwaukee - RAB turned me onto these - they're spectacular. $10 per bottle or $50 for the 6-bottle sampler (this is what I'd recommend getting) at Binny's. The Cherry Bark Vanilla and Blackstrap are great with whiskey drinks (I tend to default to Cherry Bark Vanilla for bourbon-based cocktails, and Blackstrap for rye-based), and there are few citrus-y cocktails out there that don't benefit from 4-5 drops of Jamaican #2.
Bittermens out of Brooklyn - I first learned about these from the manger of Frog N Snail in Lakeview East...she's really into cocktails and cool/unique spirits & bitters. They're using Bittermens Orange Cream Citrate in the Upper East Sider cocktail, and may have a couple other Bittermens varieties behind the bar, because she was really pumped about their bitters and was eagerly awaiting the day they become available here (until then, you can order them from Amazon, among other online sources).