Usgard is one of the newer OS/Shells available. It is my personal favorite because it is not only zshell compatible, but because the majority of new games being developed for shells other than Zshell are for Usgard.
Programs developed for usgard are usually smaller, faster, and in my opinion better. I have seen some great come out already such as Galaxian, Balloon, Orzunoid, DStar, and Solytare. There are also some games that are soon going to be out such as Aftermath and Abysmal Descent.