My Fave: Markup
Markup lets you draw on any webpage with a variety of tools to express your thoughts, make a point, or share ideas. Once you Get Markup, you can draw any time by clicking your bookmarklet or using your native browser extension.
The easiest way is to get Markup is to drag the bookmarklet to your bookmarks bar, no downloading required.
Once it’s published, you’ll have a unique Markup URL that you can share. Anyone that has a markup’s unique URL can view it. You can even reply to a received Markup.
Find out more on Markup.io