BrainPopJr is an extremely engaging website that provides games for students in all subject areas. It is easy to differentiate. It also gives many teacher resources which can help link this material to the traditional curriculum. Trial available.
WallWisher is a website that allows anyone to create a wall that is based around a topic or question. Other users can log on and leave virtual sticky notes with comments about the topic. Great for engaging students. WallWisher can address questions or topics as high on Bloom's Taxonomy as the creator chooses.
A very potent, multi-media organizational tool for constructing arguments - based on Clarkson's box. Easy to use but somewhat abstract for younger users.