Recs.
Updated
Open-source split keyboard based on the Kinesis Advantage's layout. This keyboard is available as a DIY project. Some commercial clones exist.
Specs
Pros
Pro Ultimate customizability
You can map any key to any position easily using a simple GUI, including making hardware key layers. And, since the firmware is open source, you can modify it to do anything you want. Many of the hardware components can also be altered/replaced by equivalent parts.
Pro More actions for the thumb
Unlike other keyboards, like the Lexmark M15 and Cherry G80-5000, where the only action performable by the thumbs is punching the spacebar, ErgoDox has extra keys placed near the spacebar, within thumb reach. This prevents the thumbs from being a bit redundant.
Cons
Con Needs to be assembled
This can be fun, but some people just want a working keyboard without having to spend three hours with a soldering iron. It's tricky to find pre-assembled ErgoDoxes, and they're not mass-produced. It is also possible to commission an enthusiast to build one up as well. Some Massdrops of the ErgoDox kit did came with an option allowing one to choose whether to have it pre-made or not.