Reddit: Creating a subreddit at reddit.com is an easy way to implement a communication/collaboration platform whereby participants can submit ideas and relevant info which may then be voted up or down based on perceived relevance and utility. To the extent that the judgement of participants is sound, the most useful data will rise to the top and thus be more readily accessible; this is an example of a crowd-sourced information filter.
IRC: IRC provides for an easy-to-use method of realtime communication. Download a free IRC client and join irc.freenode.net or another server of your choice. Type /join #(name of room you'd like) to join and create a new room. Invite participants and prospective participants.
Free Mind: Free-to-use mapping software useful for organizing information of all sorts, visualizing relationships, etc.
Gliffy: A web app diagram software, also usefull for organizing information and sharing documents in its cloud storages with friends.
Wikis: Setting up a wiki is a good method by which to compile information in a useful way that is easily accessible to the public.
Blog: One good option for both taking in and putting out information while also providing a central node for organization. Remember to think of a blog in terms of its actual functions, rather than its general associations - administrators can post some element of data and others may attach data below via comments.
Etherpad: typewith.me and Anonpad.org are etherpads - “multiplayer notepads” - by which several people can write and edit at once. One purpose for this is to head the document with a call for ideas on a particular subject and then drop the link in some venue where potential participants are assembled. Groups of participants can quickly produce press releases and guides or compile information on a subject, such as a company or individual.
brief.ly: Excellent way to aggregate links in a readable form. Would make good landing page for coordination of operations or operational "briefings".
http://tinychat.com/: Semi-anonymous video chat. Excellent for ad-hoc meetings.
http://www.chatterous.com/: Disposable web based IRC like chat rooms
http://www.chatzy.com/: Disposable web based IRC like chat rooms
http://anologue.com/: Anonymous disposable web based chat rooms UUID based
http://www.disposablechat.com/: Create disposable named web based chat rooms
http://www.babelwith.me/: UUID based disposable web chat
http://www.stinto.net/: UUID based disposable web chat
http://chats.io/: UUID based disposable web chat similar to anologue
http://jotonce.com/: password protected quick message passing
http://lettur.com/: Anonymous blogging system
http://pen.io/: Text based web page hosting; excellent for publishing documents
http://www.graffitly.com/: anonymous message passing system
http://www.ucoz.com/: free website hosting
http://www.dinkypage.com/: HTML based web page hosting
http://www.wikispaces.com/: Easy to use wiki hosting
http://www.wikidot.com/: Wiki hosting
http://letscrate.com/: Drag & Drop file sharing
http://ge.tt/: File sharing at it's best; has a realtime filesharing option on by default (this means you can upload files and someone can download while you are uploading)
http://chi.mp/: Personal information aggregator. Recommended use it aggregation of other content
http://pastehtml.com/: Anonymous web hosting (HTML and some mark-up supported)
http://www.mirrorcreator.com/: Send files to multiple file-sharing sites at once
http://jetbytes.com/: Realtime file-sharing
http://www.thiss.is/: allows you to create a site to argue anything, useful for testing ideas
http://securepastebin.com: encrypted pastebin
http://www.icerocket.com/: social networking search engine
http://pdfmyurl.com/: turn any webpage into a PDF instantly. excellent for capturing information for posterity (in case it gets taken down for example)
http://twich.me/: Web based IRC like disposable chat rooms
http://www.cowurl.com/: Personalized URL redirection good for creating faux domains for projects or acting as a project entry point.
https://join.me/: instant web based desktop sharing (supports Win32 and OSX)
http://icanhazip.com/: easy way to determine your current ip address
http://paper.li/: create a "newspaper" from public social networking sources good for monitoring activity
http://www.instapaper.com/: save a web page, read it later
http://www.fakenamegenerator.com/: generate shallow cover identities (not good for long term operations, will not provide deep cover)