Wikis have for a longtime been my favourite for collaborative ideas building. The main weakness seems to be the linear word-process driven approach to ideas capture. Compare this to real world face to face brainstorming sessions where people get out the flip charts, white boards or if you’re really luck the back of the restaurant menu to illustrate their ideas.
Mindjet, one of the leaders in the mind mapping field, have now produced a web-based version of their standalone mind mapping software mind manager. Mindjet Connect allows you to co-edit and share maps simultaneously online in realtime. You can trial it free for 30 days. The pricing information says that it is sold through subscription only on a per annum basis e.g. Mindjet Connect standard would cost roughly £300-£900 / year for 3-9 users.
Relatively new to me is mindmeister, a social networking tool that allows you to create, edit and share mind maps. It offers capability to start an online brainstorming session where people can simultaneously work on the same mind map. There are some interesting features that seek to exploit other useful tools e.g. you can initiate Skype calls directly from your brainstorming session.
Following a quick trawl around to see what’s out there I have put together a mindmap of some of the tools that you might want to try.