It's been a while since I've come across a really clever, easy to use freebie, but Wallwisher definitely fits the bill! It's a really clever site designed to allow users to post (sorry) post-its on a 'wall'. I'm currently usnig it to gather ideas from staff for content on some new curriculum papers and it's working really well. It's incredibly simple to use, it only allows 160 characters, so people have to be pithy and get to the point, and it allows viewers to see all the ideas at a glance - no clicking, no scrolling. The site designers suggest using it for party invitations and the like, but I can see that it has great potential for students' pre-learning; brainstorming, reflecting on the lessons, and the like. It would even work well for primary school children.