SpiderScribe is an online mind mapping and brainstorming tool. It lets you organize your ideas by connecting notes, files, calendar events, etc. in free-form maps. You can collaborate and share those maps online!
Our public service goal is to make the best arguments on all sides of any public debate freely available to all and continuously open to challenge and improvement by all.
"Cacoo is a user friendly online drawing tool that allows you to create a variety of diagrams such as site maps, wire frames, UML and network charts.
Cacoo can be used free of charge."
CmapTools (free, of course) allow for the developing of concept maps and you can link to images, videos, documents, etc. Not quite as flexible as Inspiration (which is my favorite, but it costs money), but a good option.