I haven't been able to access Sketchcast.com, and I wanted to demonstrate how to create a "mathcast" to a group of teachers. Then I remembered the Jing Project. This free software from TechSmith allows you to capture your screen-- either pictures or video-- and then upload your captures to TechSmith's Screencast.com website. You have 2 GB of storage for FREE, and Screencast includes embed code so that you can display your screen captures on your blog, wiki, or website.
Those teachers that have SMARTboards have the Notebook software to use to create their mathcast. If you don't have a SMARTboard you could use Microsoft Paint and your mouse to "write" with, although it's not as easy and will require some practice.
I also saw QlipBoard today, but haven't tried it yet. It may also be an option for screen captures.