General vocabulary

Non-stop English (choose a category and a level - lots of interactive activities)

Oxford Word Skills Online (choose the level and subject - various tasks, including crosswords)

Word-surfing (for teachers & learners - activities & links etc for learning vocab)

ITESLJ (Internet TESL Journal - lots of crosswords, choose your level first)

A4ESL (Vocabulary quizzes - a variety of levels)

English Works! (Gallaudet University, Washington DC - help, advice and practice)

SMIC (Dutch site - mainly vocabulary activities, but also grammar and reading)

English Page (choose the theme you want to study - word lists and definitions and follow-up activities)

 

Slang and idioms etc

Slang City (US Slang dictionary)

English Daily (general vocabulary, idioms, proverbs and lots more!)

Tower of English (links to various sites to learn idioms etc)

Learn English Today (idioms and activities & lots of other vocabulary too)

Using English (idioms and expressions)