Here is a list of prominent sub-themes that are based on Omega 4:
(Note: This list is by no means comprehensive. Additions are welcome.)
Omega 4.x based demonstration theme. Serves as a best-practice reference for the Omega documentation.
The goal is to already be fully in line with the D8 CSS Coding Standards. While Omega 4.x core will do the heavy lifting of converting the existing D7 core CSS and templates to that standard (and override the corresponding template and CSS files from Core and some major contrib moduiles) we will build the CSS and templates specific to our demo/showcase theme according to those standards as well.
Ohm has been included as part of the official Omega 4.x theme package since omega 7.x-4.0-beta5 (2013-07-13),
Gridly is a minimal & responsive Omega 4 subtheme. It is distributed as an install profile with all non-visual functionality provided as features. The theme is ideal for graphic designers or photographers or people who wish to publish a very visual/photo orientated website.