So often I find that students have trouble identifying key signatures. With musictheory.net they have plenty of exercises and lessons that students can use to practice their skills. The one great thing about about the program is that I can customize the exercises to meet my students needs. The students can even go in an create their own exercises. Since the middle school curriculum only covers up to four sharps and flats for key signatures, I can create exercises that only cover these keys and leaves out the other keys. Of course I can add them back in if the students have mastered the current required content. Go here to see my customized version. I've also added this screen shot below so that you can see what the exercise looks like.
The students simple click on the key that they think is represented and the program tells them if it is correct or not and also keeps a tally of how well they are doing. If the student doesn't know what the key is, they can simply select the "reveal answer" button to get the answer.