Two of our favorites below do more than just allow students to post. They also allow students to post pictures (either from the internet, camera phones, or illustrated) and GIFs! We are really big on GIFs so that's a huge deal to us.
Padlet- As a teacher, you can make a padlet site and give the link to students to post. Students just double click to post a sticky anywhere! Students do not have to have an account to post, but you can turn Padlet on "approval only" if you want so that you can approve posts before they show up.
Linoit- As a teacher, you can make a Linoit site and give a link to students to post. LinoIt is fancy in that it has different color sticky notes available and TONS of room to post all over the board (notice the navigation area in the bottom right).