Hot Potatoes

Hot Potatoes is a quiz maker.  You can download it for free from here, and it has a range of different types of quiz, ranging from a crossword, to multiple choice questions, matching activities and clozes.  It's nice in that you can produce quizzes that are applicable to your learners and include colour, pictures, and useful feedback with the answers.
This picture shows you the screen you get for making a JQuiz:

It's a great tool if you have something specific that you want to test your learners on, and they can be saved to use with further classes or to share with colleagues or on the net.  It's worth considering, though,  that they take a lot of time to make and a very short time for the learners to consume.  Also, although you can include feedback with the answers, on the whole, they are a testing, rather than a learning tool.  They do take a lot of time to develop and there is already a huge amount of quiz type material on the net, so if you want to test your learners on standard grammatical items, it's probably best to do a quick Google search.  If you want to develop more specific quizzes, though - this is your tool.