General English grammar sites for English Learners
http://egg.ust.hk/cl3/index.html
http://www.englishpractice.com/topics/grammar/
http://web2.uvcs.uvic.ca/elc/studyzone/grammar.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
http://public.wsu.edu/~brians/errors/errors.html
Corpus-based grammar exercises