I first introduced Iknowthat to my students several years ago when it was available for free with advertising. A free version of the site is still available but with limited content and the advertising remains. I signed up for a teacher account two years ago and we are now using it with all of our grade 4 classes. The paid version is not necessarily cheap for teachers on a limited budget, but it removes the advertising, unlocks all content and has great tools for both teachers and students alike.
The site organizes content by both subject and grade level. I personally prefer to view it by subject since you can see all content available. The Language Arts and Math sections are particularly useful, with lots of fun activities which cater to different topic areas.
One of the perhaps less know features of the site which students love is the quizzes worldwide. Students can play against each other (or alone) using quizzes created by others, their teacher or even by each other. This has proven a great tool for reviewing content or practicing for a quiz. If you do not have many computers in your classroom, you could choose classic mode which allows you to create teams and do the quiz show on a single computer. This is also a lot of fun if you want to do it as a whole class activity.
Some of the games in the subjects section also allow students to play against each other but there is a limited choice of games that allow this functionality.
Students also enjoy the Bee Hive which is a safe virtual world where students can use points they accumulate by completing activities, to decorate their own hive.
I have mixed feelings about allowing students to access this since they can spend too much time here. Luckily, with your teacher account you can choose to turn off this function when you need to.
With your teacher account, you can not only add your students, but can also see what activities students have been doing and monitor the progress of each student in the class. The results are not displayed in traditional grade form but in a way which gives you an idea of whether or not students are engaging in the activities.
If you have not used this site yet, it is worthwhile to give it a try by signing up for a free trial. If you decide that the site is appropriate for your students, you can switch to the paid version which is paid on an annual basis.
I have found that Iknowthat is a great resource to have available to students, both in the class and for extra practice from home. It is a site which is heavily Flash based so will not work on devices which do not support Flash. Fortunately, they do have some paid apps available ($1.99 each) on the App Store.
If you are looking for a single source which is fun and kid friendly, Iknowthat is worth checking out.