English for Everybody
Attractive site with free courses, examination practice (KET, PET, FCE, CAE, CPE & TOEFL), reading practice and language games. Uses crosswords, colour illustrations, multiple choice, interactive gap fills etc.
http://www.english-online.org.uk/backindex.html

Englishjet
Well laid-out site offering a series of graded tests from Elementary level upwards and including some relevant exam practice for First Certificate (word order in sentences, sentence transformation etc).
http://www.englishjet.com/english_courses_files/tests.htm

Learn English Today
Quite a comprehensive site. Offers practice and reference materials that are accessible and relevant. Includes exercies, idioms, puzzles, Business English and an extensive grammar reference and practice section.
http://www.learn-english-today.com/index.html

Churchill House Online English
Easy-to-navigate site offering a variety of lessons and tests for learners of English and different levels. The site includes graded tests, phrasal verb activities, gap-fill activities, drag and drop exercises, word order etc.
http://www.churchillhouse.com/english/learners.html