Twiddla is one of the better collaborative whiteboards I've encountered. Twiddla not only allows you and students to collaborate on a whiteboard in real time, but you can upload and work on websites, graphics, photos, or worksheets. Once you start a meeting you can invite collaborators with an email or simply handout the meeting number with a QR code. With or without an account, you can take advantage of Twiddla and collaborate with desktops, laptops, or tablets. No downloads or plug-ins are required, the site is totally web based. If you do create a free trial membership you can request a free educators account by emailing the site from your school email account.