The BBC is possibly one of the richest sources of high-quality, free English learning material on the Internet.  Their Learning English site has so many sections I really can’t talk about all of them here (although I will say Words in the News is a very good place to start).  Skillswise has excellent sections on spelling, reading, and writing.  Their Video Nation site has interviews with real people and a short quiz afterwards to see if you...

The Guide to Grammar and Writing is a resource for American college students so the language on the site is considerably more advanced than on a site for English learners.  However, the information is extensive and you may find the pages on punctuation (under Word and Sentence Level) and sentence combining skills (under Paragraph Level) particularly useful.

English Grammar Online 4U is an excellent writing resource that provides a lot of very useful tips without getting too complicated.  Their General Information on Writing English Texts is a very good place to start improving your writing.

The Purdue OWL (Online Writing Laboratory) is a huge reference to improve your writing.  They have a large section of grammar and punctuation rules for native speakers for your most difficult grammar questions. About.com has a lot of resources although it may be difficult to find what you are looking for if you don’t know the correct terminology.  Use your textbook or another grammar reference to identify what you are looking for and then...