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.
This is an interesting site in which you can type a verb, then choose the tense you'd like to have exemplified, and it will give you real results pulled from news sites all across the web. You can even edit the results within a topic, i.e. business, food, education.
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!
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!
English 4 U
Many, many online exercises, graded in easy, medium or difficult.
Lots of links to verb tenses, with contrasts. For the explanations, visit another page: https://www.englisch-hilfen.de/en/grammar_list/zeitformen.htm
ESL Blues Trivia Grammar quizzes
This is a fun set of trivia quizzes based on different verb tenses and voices