Virtual Cork Boards

Lino: is all about collaboratively placing photos and sticky notes on a board, great for planning!
Real Time Board: Robust, collaborative shared board with linking to Google Drive, and much, much more. Currently in beta
Note App: Sticky note sharing

Online Polls

AnswerGarden: is a way to get feedback on a blog or site by asking a question, and responses fill a space in a type of tag cloud
Socrative: Much more than an online poll, Socrative allows teachers to engage students with quick quizzes, games, questions, and other items using their mobile devices...getting real time feedback
Active Prompt: image based create or choose an image, students vote by placing a dot in the location, and you can see a heatmap of responses on the image

Mind Mapping

Murally: allows collaborative pinning of images, video, text, and also allows basic presentation mode on a community board
Lucid: Mind mapping, flow charts, etc. free educator options
Slatebox: Mindmapping tool like Popplet but a bit more technical
Pearltrees: Explore related interests with others on the web by connecting pearls and creating trees
MindMaple: Another visual mind mapper
Trello: is all about project planning in a visual way
Processon: a collaborative mind-mapping space with adjustable privacy and group settings

Shared Whiteboards/Meetings

Stoodle: Use your mic, and no account creation is required. From the great folks at ck12! Click "launch a classroom," name your room, and share the URL assigned to your room
Flock Draw: This is a painting & drawing tool. It makes it easy to draw online free with multiple people for fun or business. You can have unlimited people in a room
Twiddla: A no-setup, web-based meeting space (free pro accounts for teachers)
Screenleap: Allows instant screen sharing to any browser, with no downloads or sign-ins

Social Networking

Twiducate: A simple socialnetworking site for teachers and students (no emails for students required)

Video Collaboration

WeVideo: Amazing, collaborative video editing
Wirewwax: Tagable interactive video tool

Quiz/Practice Creators/Graders

Jeporady Labs: Create your own Jeopardy Game with these great templates (great for IWB or Projection)
Flubaroo: Grade online assignments in a single step
Textivate: Creates a range of interactive browser-based activities based on any text you put into a textbox. (Texts of up to 500 words and/or up to 100 matching items)
Equizshow: Build templates and quiz shows on any topic