Usability 
WPMU is a great resource for anyone who runs a WordPress site. The site's name suggests that it's about the, now-depreciated, "MU" multi-user/mult-site version of WordPress. It's not, WPMU.org is the largest, most popular general WordPress specialist site that's not part of WordPress.org itself. Packed with WordPress lore, news, tips, tutorials and theme and plug-in downloads that should be of interest to anyone who runs a WordPress or BuddyPress site.
WPMU.org uses a "mobile-first, responsive" style which means that the site adapts itself, using mainly front-end or client side logic, to a wide range of browsers, both desktop and mobile. In a post about the design, one of designers talks about howthe "mobile first" approach means that mobile devices don't have to download heavy graphics and other resources intended for desktop browsers.
The design targets devices ranging from modern mobile browsers with a screen width of 320 px or more on up to the desktop. I tried it in the Android Gingerbread, Opera Mobile 12, Opera Mini 7, Firefox Mobile 12.0, WebOS 1.4.5.1 and Symbian Belle mobile browsers and with Chrome, Firefox and Opera on the desktop. The site looked good and worked well on all of them which I find pretty impressive.
The mobile first approach worked pretty well too. according to Opera Mobile the page was a relatively svelte 247 KB while in Firefox on the desktop it was 1184 KB.