A wiki is a website where users can add, remove, and edit every page using a web browser. It's so terrifically easy for people to jump in and revise pages that wikis are becoming known as the tool of choice for large, multiple-participant projects.
Wikipedia builds on transparency, simple linking, and a low barrier to entry for crowds of people to be involved in editing and authoring. We can use these same qualities with just two or three people for a different outcome: a shared workspace and, in effect, a shared memory.