I have been using Linoit quite a bit lately. It is an excellent alternative to wallwisher which seems pretty finicky lately.
Teachers have been building canvases on linoit to review fraction concepts, post assignments and comprehension questions for novels, as a "due now" and as an "exit ticket" for many different topics.
You can be notified when a stick is posted on your canvas if you would like and have the ability to make the canvases as private or public as you like. I also like the daily digest I receive for each canvas, letting me know the action that has taken place and any tasks that are due. You can upload videos, images, and documents to your canvas. You can even post tasks with due dates and emoticons.
Here are some examples:
I like to use goo.gl or tinyurl to shorten the links to the canvases, making it easier for students to access if a teacher does not want to hyperlink it from a blog or web page.
People have also suggested the similar, corkboard.me, which I hope to try out soon.