One quick note: I tried to select something on this page and nothing would happen. I finally figured out that I had to sign up for a service first.

Have you considered creating a bit of javascript that any site can embed on their site that would allow a visitor to annotate? This could be a really powerful channel for getting sign ups. Especially if you partner with some quality sites.

Also, have you considered making it anonymous to start like github:gist? It would be great to test it out without making a commitment of signing up. I.e., "Do I really have to sign up for something, before I even know if it will provide value to me?"