As a self-proclaimed HTPC addict, I've become more and more interested in the many different User Interface layouts and themes for the different software alternatives. Themes and Skins are partly about the appearance and visual appeal, but often times it's also about the UI (user interface) and how you actually navigate your HTPC program.
Plex, a fork of the XBMC HTPC software has some of the more exciting themes/UIs available. Below are several screen-casts of 3 themes available to Plex users. All three look very, very nice and display the awesome capabilites of the HTPC. What is even more amazing to me is that this creativity and ingenuity of themes you find at Plex, SageMC, XBMC, Meedio, Media Portal and others all are coming from unpaid developers who just love their HTPCs.
I think most if not all of the themes available for Plex are also available for XBMC (probably originally created for XBMC matter of fact.)
This skin/theme is called Aeon and is also available on XBMC. This skin definitely has the cool factor going for it and is VERY popular:
Plex with Aeon skin demo from muzo178 on Vimeo.
Below is the MediaStream skin - The UI is unique from many other HTPC UI's I've seen and I really like how this one works:
Plex with MediaStream skin demo from muzo178 on Vimeo.
via Plex Wiki