Grammar Safari
http://deil.lang.uiuc.edu/web.pages/grammarsafari.html

LinguaCenter's (U of Illinois, Urbana-Champaign) page with various activities to help you with grammar.

Guide to Grammar and Writing
http://cctc2.commnet.edu/grammar/index.htm

A short page offering good examples of shifts in verb tense and "things we have to remember about pronoun reference," along with links to quizzes and other grammar resources.

Big Dog's Grammar: A bare bones guide to English
http://aliscot.com/bigdog

This site covers the following grammar points: Subjects, Verbs, Prepositions, Fragments, Comma-splices/Fused sentences, Joiners, Agreement (Subject-verb and pronoun-antecedent), Dangling and Misplaced modifiers, Parallel structures, Reference, Pronouns, Consistency, Active/passive constructions. Interactive exercises allow the reader to see how well s/he has understood each point (except for Joiners).