For a quiz on formal letters:
http://www.bbc.co.uk/apps/ifl/worldservice/quiznet/quizengine?ContentType=text/html;quiz=1121_formal_letters
If you fill in the quiz correctly than you have understood a few things that belong in or with a formal letter! ( It is optional to do the quiz)

For a list of business vocabulary, useful phrases and terms:.
http://www.learn-english-today.com/business-english/A-business-english-contents.html

For an explanation of a persuasive letter and for examples:
http://www.buzzle.com/articles/persuasive-letter-example.html

For a guide to write a persuasive letter:
http://www.brenthodgson.com/how-to-write-a-persuasive-letter

For more examples of letters:
http://www.bestsampleletters.com/

If you want to know strategies for writing a persuasive letter:
http://www.bestsampleletters.com/

Lijst met de meest populaire popartiesten:
http://www.123musicstars.com/

Beste filmacteurs en -actrices
http://www.buzzle.com/articles/best-actors-of-all-time.html