General English grammar sites
Designed for Chinese students but a very useful general guide
http://learning.cl3.ust.hk/english-grammar-guide/frame.htm
Designed for native speakers
http://grammar.ccc.commnet.edu/grammar/index.htm
http://grammar.quickanddirtytips.com/
http://www.grammaruntied.com/index.html
http://www.englishpage.com/weeklylesson/weeklylesson.html
http://opengrammar.sentenceworks.com/tiki-index.php
Common errors
http://www.writing.utoronto.ca/advice/style-and-editing/hit-parade-of-errors
http://www.writing.utoronto.ca/advice/english-as-a-second-language
Corpus-based grammar exercises