There are many useful websites for learners of English. Here are links to just a few of my favourites.
Spelling and Punctuation (Basic Skills level 1 and above): BBC Skillswise Click 'Words' to practise spelling and punctuation skills through some fun games and quizzes.
Vocabulary (ESOL): Phrasal verbs at Dave's ESL Cafe an A-Z list of phrasal verbs with definitions and examples
Grammar (ESOL): EnglishPage.com Click 'Verb Tenses' for some useful reminders and practice exercises on all the tenses for intermediate/advanced students (there's lots more on this site too - check it out!)
Reading: EF EnglishTown's 'Hello' magazine short up-to-date news stories with some words explained to help you understand them.
Pronunciation (ESOL): Fonetiks Listen to the different sounds of English (in different varieties of English!)
e-pals (ESOL): EF EnglishTown's community When you visit for the first time you must register (click 'Become a member now') and then you can click 'pen pals' and find friends to write to by email.Easy quizzes: Simple vocabulary quizzes with pictures. Part of Charles Kelly's Interesting things for ESL Students
Harder Quizzes: EnglishClub.net Fun Quizzes
There are many more sites for learners of English on the web. Find many of them in the Web Guide Links at Dave's ESL Cafe and in the ESLoop.