A quick survey of LARP writing tools I know about (including some I have never used). I put these in alphabetical order and added some that other gamers told me about. If you tell me about another one I’ll add it to the list. 🙂

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Stuff designed by LARPers

Akkar: this is from some folks in Norway and from what I can gleen from their page it is a web service you can install on your own server. I have never used it.

http://akkar.org/?view=&lang=en

Créa’GN: I think it’s an online collaborative tool, but the whole site is in French, and regrettably I don’t read French. (It looks like the site may require a paid subscription for writers? I am having difficulty reading the page, so I’m not 100% sure on this.) I have never used this system.

http://www.creagn.com/

GameTeX: a LaTeX based system for writing LARPs. If you’ve spent any time writing academic papers you probably either love or hate LaTeX (it is very powerful but not very user-friendly).
Edit: You can now get a copy of GameTex from http://web.mit.edu/kenclary/Public/Guild/GameTeX/ or a newer fork on https://github.com/dwhalen/SGSTeX
You may also find this tutorial for getting it working on a Mac helpful:
https://metaparadox.dreamwidth.org/1214.html

Gender Swap: a tool for managing gender changing LARP characters to match casting needs, by me. This is not a full system, just a markup processing tool for gendering characters at casting time. It supports standard pronouns for male, female, they-neutral, and ze-neutral genders.
http://www.digitalchangeling.com/gender_swap_pg/

Larpwriter: looks like a web page based cloud system that can generate PDF sheets. I know nothing other than what’s written on their page.
http://www.larpwriter.com/en/

NIMS: This is a web page based LARP writing tool with either an online or offline version available. It used to be Russian language only but has recently had an English language release. I have never used NIMS.
http://trechkalov.com/index-en.html
A Tutorial for using the system: https://www.youtube.com/watch?v=yxGeTzGlCiw

Querki: another online cloud service based system by Justin du coeur. Alan saw a demo of this one at Intercon and was super impressed with it. I have never used this system.
https://www.querki.net/

Spindle: also a cloud service system, and what I know about it is pretty much what it says on their IndieGoGo page. (As a spinner who uses a drop spindle, I <3 their logo. 🙂 )
http://spindle.pl/
https://www.indiegogo.com/projects/spindle-a-simple-solution-for-writing-your-larp#/

Vellum: a collaborative writing tool developed by Nat Budin. It’s an online service based system. I have never used this system.
http://vellum.aegames.org/

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Stuff for general collaborative writing

Any Wiki System: Any wiki system you like will probably work fine for collaborative online editing as well. I find wikis tedious for small to medium games but some people love them. I’m in the middle of working on a game with Kathleen Leeds De Smet and Katie Zenke via a private wiki.

Dropbox: a cloud storage system for files that can be used to share collaborative projects. When used for LARPs there is the caveat that the version of Dropbox that functions in your file system can’t really handle concurrent editing of a given document (I believe their web system can). The free version of their service offers more than enough space for most writers to store many games. I’ve used Dropbox to write several games collaboratively.
https://www.dropbox.com/

Google Drive: more cloud storage of collaboratively edited documents! Google Drive is great for letting multiple writers concurrently edit documents, but you are stuck with their document format (the exporting capabilities aren’t terrible, but once you export you don’t get to edit collaboratively any more). Managing file organization is a nightmare because they have no tools for anything more complicated that “copy this file.” It does have a nice editing history to let you review other author’s changes. I’ve used Google Drive to write games collaboratively.
https://drive.google.com

OneNote: a cloud storage system for notes that lets you share and collaborate with others. Katie Zenke tells me this works well for working on games alone or with other writers. I haven’t used OneNote.
https://www.onenote.com/

Scrivener: a writing tool intended for other forms of writing like novels, but can also be used for LARPs. Julia Ellingboe tells me that there are LARP specific templates available for Scrivener and their site confirms that it will hook up to various cloud services like Dropbox to allow for multi-writer collaboration. I’ve never used this software.
https://www.literatureandlatte.com/scrivener.php

Source Control Software: Those of us who are programmers will be familiar with various source control systems like Git or Subversion. To use them for collaboration you need all your writers to be comfortable using something a little more cryptic and technologically confusing. Some source control systems have only command line tools and others have relatively easy to use visual clients. Git is notable because of the existence of GitHub, which will allow you to have a centralized online place for writers to connect their copy of a git repository so you can share your writing easily. GitHub is free, but the things that you put there are (by default) visible to anyone on the web. You can give GitHub money to get a private place to put your stuff. I’ve used various source control systems at work and in my personal collaborative projects.

If you can think of any others I should add leave me a comment. 🙂