English Grammar Secrets
englishgrammarsecrets.com is an excellent website for practising English grammar. Choose from the list of grammar structures to see an explanation with examples. Then click on one of the exercises to practise. What’s great about the exercises is that they are all interactive; drag and drop, gapfill, matching.
The only complaint I have with this website is that there are lots of ads, but the site is free. You can always install an ad blocker to hide the ads.
Update: The website appears to have been updated so that it no longer requires the Flash plugin, although it is sometimes a little temperamental.