Haris had posted the following tweet few months ago:
After quickly checking out grammati.com, I had signed up. Received the following email this morning (for some reason Google decided to mark this as spam for the first time) and I thought it was time to spread the word:
Basically, the nifty little service sends you a daily mail, such as above, and asks you to answer 5 simple grammar questions. In my 100 days of usage, I have come to love the service and find it a great way to remain a grammar nazi (quick apology to friends whom I have annoyed time and again). Specifically, I found I could easily unlearn some of my frequent grammatical mistakes and learn subtle difference in usage of phrases with similar meaning like in case vs if.
This is quite an effortless habit to form as it takes less than a few minutes of time and there is no chance you can miss it (if you check your email regularly). Highly recommended, go sign up at Grammati.com.
Aware of any other such services, feel free to leave a note in the comments.