As more and more schools look towards integrating the iPad and iTouch into their classrooms, the range of educational applications available is growing. For specific apps that may be helpful for students with disabilities, you may want to check out iPhone, iPad, and iPod Touch Apps for Special Education, an extensive list compiled by assistive technology specialists and helpfully broken down by category (communication, math, writing, music, art, etc.). For another view of how the iPad might be beneficial for students with disabilities, The iPad: a Near-Miracle for My Son with Autism chronicles one mother's use of assistive technology and educational apps with her autistic son; she has some great suggestions and videos of her son using different apps.
For older children, apps like The Elements are exciting examples of what is possible with the iPad, as students can explore the Periodic Table in an interactive, media-rich and engaging way. Penultimate is a popular note-taking app that students may enjoy; students may also do well with fun games that teach math skills, such as Alien Equation. Apps for astronomy, Star Walk and Solar Walk would also be good choices for older students. BrainPop has just released a free app that delivers a new featured movie every day, teaching students about a wide variety of topics.
There are so many educational apps available, with new ones coming out every day, that it can be hard to keep track of them all. You may want to check out reviews of educational apps from other teachers to help you find those that are worth checking out for your students. I Education Apps Review has a collection of reviews from teachers that can help get you started.