WPCandy’s Completely Unofficial Guide to Plugin UI, published in January this year by Daniel Immke, who takes us through some of the user interface elements, used in WordPress themes and plugins. Navigation, Settings API, tabs, the Toolbar, Dashboard (and non-dashboard) widgets and more.
Also worth mentioning an earlier post by Ryan Imel (founder of WPCandy) called Custom designed WordPress options screens need to go and Otto’s comment. That’s three links in one post, sorry! Have a great weekend :)
Thanks to @maorhazan for the reminder.