Besides articles about Plugins, Themes, Wplift does also offer Guides and Tips to get difficult jobs done in WordPress at an easy way. There is also a News section and an Interviews section, which I personally find interesting, because there is a lot of information about making a living with WordPress. A lot of the articles cover unique subjects which add value to the WordPress community. The expert level of the articles goes from simple to complex and new articles appear almost every day.
WPSnipp is a handy, searchable code snippet reference site for WordPress developers, currently publishing 610+ snippets. Every developer works through the same concept: find as much as reusable code as possible. This website really enables that concept!
This extended category of SmashingMagazine.com features quality articles about developing clean, smart and fast websites with WordPress. The articles are intermediate level, with an emphasis on practical, hands-on discussions related to WordPress