Popplet This paid app lets you collaborate and brainstorm with another popplet user in real time. You can download diagrams that you have created on the web to share and collaborate. I wish the free Popplet Lite version allowed for collaboration.
Diigo This app allows you to install an applet for the Safari browser to bookmark resources. The app gives you access to all your saved bookmarks. Search for tags created by classroom groups, and this app becomes collaborative. The app is supposed to get group bookmarking in a future update.