If you a document or picture that you want to share and discuss with a group of people, you can use
http://www.twiddla.com and groupboard.com
Twiddla is a no setup, webbased meeting place. On the site you can “Mark up websites, graphics, and photos, or start brainstorming on a blank canvas. Browse the web with your friends or make that conference call more productive than ever. No plug-ins, downloads, or firewall voodoo – it’s all here, ready to go when you are. Browser-agnostic, user-friendly.”
Groupboard is a free online shared whiteboard and chat app that can be easily embedded into your website. It works on any web browser including iPhone, iPad and Android with no downloads or plugins required. Groupboard has been used by universities, online tutors, architects, designers and artists since 1998 for distance learning, collaborative design and online doodling!
Both Twiddla and Groupboard also great tools for younger kids since the tools allow kids to write using brushes of different sizes and colors.