Grammar

the Grammar aquarium
http://perso.wanadoo.es/autoenglish/freeexercises.htm

from a German web site - hilfen
http://www.englisch-hilfen.de/en/exercises_list/alle_grammar.htm

English 4 U
http://www.english-4u.de/grammar_exercises.htm

Placement tests

The test should take around 45 minutes to complete. At the end of the test you will see your test result and be given a percentage. You will also be able to follow a link to a page which will estimate your level compared to several international English exams (PET, FCE, CAE, CPE, IELTS, TOEFL); and the Council of Europe language assessment scale.
http://englishenglish.com/englishtest.htm

Graded English placement test - Cambridge English readers - vocabulary
http://www.cambridge.org/elt/readers/placement_test/RPT_last.swf

Basic placement test
http://www.tolearnenglish.com/test-de-niveau-anglais-grammaire.php

placement test - to test your ITALIAN
http://www.cyberitalian.com/en/html/placement_test.html

for children

British Council - very nice activities
http://www.britishcouncil.org/kids.htm

Frog - past verbs
http://www.english-online.org.uk/games/pasttense.htm

quick click bricks
http://www.english-online.org.uk/games/gamevocab.htm

the description detective
http://www.english-online.org.uk/games/findPerson.htm

vocabulary jungle
http://www.english-online.org.uk/games/vocabblock.htm

English star
http://www.english-online.org.uk/games/vocabexpertp1.htm

noun, verb or both?
http://www.english-online.org.uk/games/verbnounboth.htm

for real kids
http://www.eslkidstuff.com/OnlineGamesMain.htm

English for Fliers
http://www.marks-english-school.com/games.html

Crosswords for Kids
http://notredamke.rkc.si/english/crosswords/Crosswords.htm

Nothing Ventured, Nothing Gained