Often I need to share a concept with a friend. Now that web-based whiteboards are available this has gotten so much easier! All for FREE. My favorite is Skrbl Twiddla.

Here are some sites collected from this Blog (http://techlogg.blogspot.com/2007/03/web-based-interactive-whiteboards.html)

Twiddla Real collaboration, in real time.
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, mom-approved. Oh yeah, it does one-click audio chats too.

skrbl Online Service - With skrbl (scribble) you can write notes, sketch drawings, upload pictures and files. You can switch between freehand drawing and rich text entry modes easily and it saves and syncs your work automatically.

Thinkature Online Service - is about instant messaging inside a virtual workspace. With it you can set up a collaboration environment, a meeting room or a personal web-based whiteboard or even use it as a planning, brainstorming or outlining tool.

Imagination Cubed Online Service - This is a straightforward, collaborative whiteboard where you can invite others to join in by instant messenger or e-mail. There are no accounts required and you're up and running as soon as you load the page.