My Top Favorites:
These are sites that I go back to over and over again!
My very favorite. It has great explanations, with really good verb tense contrast exercises.
You'll need to navigate into "Verbs" and then "Tenses", but this is another site with not only explanations and exercises, but good verb tense contrasts.
Perfect English Grammar
This site has three different pages for each tense: making, using and practicing. It also has some nice tense contrast explanations and exercises.
My English Pages
Very wide variety of grammar topics with brief explanations and an exercise to an exercise for each.
More Great Resources
English Grammar with Jennifer
Many of you know Jennifer from her pronunciation site, but she has some very gentle, well-explained grammar videos as well.
Lots of good exercises here; when you first choose a topic, you're directed to the quiz and then can choose another tab for "Explanation". Be aware that it tends to be more British!
ESL Tower grammar games
Lots of online grammar games. You will need to enable Adobe Flash Player!
This link is the same as above, but printable grammar worksheets oriented to teachers--a wide variety of subjects.
Learn English Feel Good
This website may not have explanations, but such a wealth of exercises--great for when your students need extra practice--great upper level resource!
Even if you don't use the Azar grammar books, this site has a lot of useful extras: powerpoints, song activities, and extra handouts.
English 4 U
Many, many online exercises, graded in easy, medium or difficult.
Nice links to exercises on other websites based on verb tense
Lots of links to verb tenses, with contrasts. For the explanations, visit another page: https://www.englisch-hilfen.de/en/grammar_list/zeitformen.htm