This is a very simple but very nice little website: http://www.oneaday.org/
It shows a different Chinese idiom every day, with characters (traditional or simplified), pronunciation, and very fluent English translations. Many traditional four-character chengyu, as well as other useful classical and vernacular expressions. You can also search or browse the archive for idioms from previous days. Oneaday can be subscribed to as an RSS feed, as well.
One of my favorites from the site:
肉包子打狗
(rou4 bao1 zi3 da3 gou3)Beating a dog with a meat bun:
to use the wrong method to solve a problem