Update on March 15, 2013: English1010.com is no longer online, and has been replaced by an ad page. This article has been removed from the Free Language English Directory and left for archival purposes, with links removed.
This site has some good lessons covering a variety of commonly-used idioms, expressions and phrasal verbs in the English language. Many of these lessons are supplemented with audio, which is handy for pronunciation and listening practice.
This site is best for intermediate to advanced learners of English.
This resource is also a great tool for English teachers who can point students to the site and have them do any number of activities regarding idioms and expressions.