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.
Pro Key rows are aligned
Almost all the keyboards have a small row offset (for example, on a standard QWERTY keyboard, the A key is not directly below the Q key. On the ErgoDox, this isn't the case: keys form straight columns. Together with the movability of the two parts you can stretch your fingers straight on, not slightly sidewards. This helps with touch typing, since you just need to stretch your fingers straight onward to reach the key above, rather than stretching it onwards and sideways.
Pro Customization settings saved on the keyboard
With most programmable keyboards, you have to use their software in order to save and reuse your customizations. Using such keyboards on other systems "degrades" them to their default setting. Since with the ErgoDox the settings are saved directly on the keyboard, no matter the system where you're using it, your macros, tap dances and layers are always available.