Grammar Blast game
Online quiz/games for teaching grammar, broken down by grade and skill. Aimed at middle schoolers:
http://www.eduplace.com/kids/hme/6_8/grammar/
Note this is the Houghton Mifflin Harcourt publisher's site, but is one of the better/most complete free resources I've found yet for grammar. The other resources on the site are so-so, so far as I've seen.