Draw it Live is a free application that allows you to work together with other people to draw in real time. You simply create a whiteboard and share its URL to allow other people to join in.
Twiddla meetings allow users to interact in real time with drawing tools, chat, voice, images, videos, plain text, documents, emails, and GoogleMaps.
"As we mentioned, the App Store is loaded with these kinds of apps, and many of them are quite good. To help narrow things down, we've rounded up five of the best ones, focusing on apps that are either free or have a freemium pay model."
"Educreations is an exciting app that transforms your iPad into a recordable whiteboard. It records your voice, handwriting and also allows you to insert pictures to produce your own personal video lessons that you and your students can share online. Your lessons are stored online and can be accessed by students on any computer or iPad. "
"Thinking Blocks is a nice site for elementary and middle school mathematics teachers. Thinking Blocks provides interactive templates in which students use brightly colored blocks to model and solve problems. As students work through the problems they are provided with feedback as to whether or not they are using the correct sequence to solve each problem. There are templates and problems for addition, multiplication, fractions, and ratios. You can also develop your own problems using the modeling tool."