I decided to throw something together this morning since so many people were complaining that chalk.37signals.com didn't work anywhere except on iPads.
I appreciate this app greatly, since I was particularly frustrated that chalk didn't work on the pc. I also think it is a fantastic reminder of how important presentation is. The two apps have two very different audiences, and their appearances have a tremendous impact on how they are received. I am not denigrating marker at all, especially because many of my experiments are just to see if something is possible, as soon as I succeed at a proof of concept, I have reached satisfaction.
Lacking any real design ability myself, I am always frustrated at how big a difference it can make in how your app is communicated both verbally and visually.
Source is available here, do whatever you want with it: https://github.com/mrduncan/marker
Also, I only tested it in Safari 5 on OS X so no guarantees that it'll work anywhere else. Patches accepted :)