Grammar

http://www.oup.com/elt/students/?cc=gb - OUP student sites

http://www.oup.com/elt/global/products/headway/ - Headway student site

http://www.englishmaze.com/main/home.html - English maze: try the "mistakes room" for common errors. There's also an "accents room" for practice listening to different English accents

http://www.bbc.co.uk/worldservice/learningenglish/grammar/grammar_challenge/ - BBC's weekly grammar challenge

http://www.britishcouncil.org/learnenglish/central - British Council Learn English Central: all sorts of English practice, including grammar explanations and games