BBC Learning English - A large collection of idioms listed alphabetically, with brief definitions.

Free weekly newsletter with a vocabulary quiz with a business focus (The quiz is often on business idioms/expressions seen in the news)

Self Study Idiom Quizzes

The Idiom Site - definitions of popular idioms

Nik Peachey's Daily Activities - tips for self study

Artilce and classroom activities on
puns, metaphors and idioms

Searchable index of idioms expressions

Online idioms and expressions board game

Vocabulary building fun and games

Weekly worksheets for colloquial expressions

Teaching idioms using movies - transcript of 'Monsters, Inc'