Tommy Spall, from my Twitter PLN, created this comprehensive list of 20 "Digital Whiteboards" for both teacher and student use. Some of these are just simple digital whiteboards. Some have a robust selection of tools and widgets that gear more towards classroom management. Finally, some are just awesome for digital post-its and classroom/team collaboration. Take a look through the resource list and find the best tool that will meet the needs of your instruction and your students.