Markup.io is a free webtool that allows you to annotate the web. You can point out, circle, and highlight any website to your liking all with the easy to install bookmarklet. Any site that you want to annotate can be done with a quick click of the bookmarklet, a new page will then appear with your annotation tools at the top of the website. You can do all of this without even creating an account.
How this could be useful to you as a teacher you ask? You can quickly annotate a website and email it out to your students just by sharing a link. It does not require a lot of extra time, and your students receive your notes right away. You could also place directions on the website so that you are not sending out multiple documents for a task. Students can also respond back with your annotations on the page. You could then view their work when they send their own "Markup" back to you.
There is one unfortunate detail about Markup, and that is the annotations will not appear on an iPad. I know that this website is still in development so they may work out those details in the near future.