Chatting about mathematics (with real-time LaTeX rendering) Do you know about some tools which can be used for online chat about mathematics? In particular, I am interested in software which would be able to render LaTeX formulas. (Since LaTeX is probably the fastest possibility to type mathematics.) 
Have you tried some of them? What are their advantages and drawbacks?
NOTE 1: In this question I am asking only about solutions how to type mathematics in chat, not about possibility to use video and sound.
(For the purposes of this question, assume that only keyboard, mouse and display are plugged to your computer.)
NOTE 2: I will post some possibilities I am aware of in the answers. But I would be grateful if someone who tried these applications could expand these answers a little more.
 A: Hopefully this is helpful; I have been programming a MathJax based online chat service in my spare time (there are some styling issues still). It is quite fast and has quite a few features. Here it is:
http://mathchat.x10.mx/
Enjoy!
A: ShareLaTeX had builtin chat with MathJax:

UPDATE: it was aquired by Overleaf.  As almost all of ShareLaTeX, it was open source, and remains open source after aquisition!
Source moved here: [backend, frontend].
As of 2020 it's integrated into Overleaf.com, and all MathJax features seem to work fine:

This is a near-real-time collaborative LaTeX editor, so this is great for chatting around a writeup/paper, but you can also create empty project or file just to use chat/commenting, and gradually "spill over" into the editor for a more flexible collaboration...
P.S. Note that for review comments on particular points of the paper, Overleaf have dedicated feature for comment threads on particular points!  These don't render math currently.

A: EDIT: This project seems to be abandoned. (At least the website does not work anymore.) If anyone has the knowledge that it has been moved to another location, please, edit this post with updated information.
TeXChat is a chat which uses Mathjax for rendering math.
I've learned about it from the website MathJax in Use.
Here's a screenshot:

You can copy a source of an individual message by hovering above it and selecting the text.
A: I work on Zulip, an open source team chat server.  It supports LaTeX using $$…$$ for inline math and
```math
…
```

for displayed math.  These are rendered on the server side using KaTeX.
A: There are scripts written by Valery Alexeev.
UPDATE: The new name for this plugin is TeX for Gmail. Now there is only one plugin which adds TeX-rendering to both Gmail and Gmail Chat. (IIRC, they used to be separate.)

Here is original version of this post:
They were mentioned in this answer: https://physics.stackexchange.com/questions/7809/online-physics-collaboration-tools/7811#7811 
GmailTeX was mentioned in leo's comment. There is also GmailChatTeX.

This Firefox add-on http://thewe.net/tex/ seems to be similar.
A: Gitter which is generally programmer-optimized (markdown, syntax highlight, GitHub integration) also supports math with $$...$$ syntax.
It uses KaTeX which is very fast but not as feature-full as MathJax

Free public and private rooms up to 25 people.
A: Instabird with this MathJax addon:

A: If you want a wide platform support, including smartphones and want a mainstream messenger lots of people already use, then telegram with a latex bot can do this. If you are not familiar with telegram, it's basically like whatsapp.
Once you have telegram, you need to add a bot (just a contact) that would render LaTeX to image in your chats. Right now I have inlatexbot, but there are also others. https://github.com/vdrhtc/InLaTeXbot
It might not be as feature reach and perfect as latex plugins for jabber clients, for example, it is not capable of inline math, it just gives you an image with your formula in chat. If you need LaTeX only occasionally, this is ok.
However, I believe telegram is the only mainstream messenger that can do LaTeX and it is also available for all major platforms including mobile. It also has a superfast synchronization between your devices via cloud (yes, you can store your history on the server).
A: http://papertogether.com is a collaborative whiteboard app with chat that supports math surrounded by backticks:
Foo `a^2 + b^2 = c^2` bar baz.

Unfortunately there is no text tool to add  formulas inside the drawing area, and the chat is a small subwindow in the corner.
KaTeX-powered so supports less constructs (e.g. no matrices).
Help, Background
A: LaTeX plugin in Pidgin: 
http://sourceforge.net/projects/pidgin-latex/

This screenshot is taken from http://sourceforge.net/projects/pidgin-latex/screenshots/124729

A: I use Lyx to type LaTeX quickly. It is a WYSIWYG LaTeX editor.
You can then use screen sharing e.g with Skype so the other person can see what you are doing on Lyx. 
Never tried the screen sharing part, just an idea.
A: Overleaf has a 'Chat' feature which is accessible in the top-right corner and is LaTeX-processed.
A: Mathim - online chat with the possibility to use of LaTeX syntax (the first result that google returned for latex online chat or latex online chat math)
http://mathim.com
Screenshot from my short experiment with this tool:

Mathim was also discussed here:
http://www.physicsforums.com/showthread.php?t=193510
http://www.physicsforums.com/showthread.php?t=196699
It is possible to copy the text of session (simply by selecting it) and put it into a TeX file.
A: 
The web interface of hack.chat renders LaTeX, and there are some pre-made rooms for math, physics, etc.
It's also open source and can be installed on your own server if needed.
A: If you use Discord, invite TeXit to your server. (Go to that webpage, click "invite" and enter the name of your server.) Anytime you put dollar signs in a post, TeXit will interpret what you say as LaTeX and will repeat a TeX'd version of what you said. Here is an example of what you'll see

Mathbot is another bot with similar features, but you need to type =tex at the start of the line, which I find harder to remember.
A: Sorry for reviving a bit of an old question, however as of today, it is now possible to LaTeX over Facebook chat, automatic rendering. There are a few things to note:
$1:$ You will only be able to view this on desktop, mobile does not support it yet.
$2:$ You must enclose your message in either \(...\) for inline math (can be interspersed in text) or $$...$$ for display math (must be the sole content of a chat message, not recognized if preceded/followed by other text) 
$3:$ There is a one-line size limit which isn't terribly generous.
$4:$ If KaTeX can't parse the math, it will stay unrendered text, but you can hover over it to see the error message.
So, a bit restricted, but there's a lot of potential, and the future is now. But yeah, again sorry for mentioning it on such an old message, but I think it's worth letting people know. 
A: Deprecated: There are easier ways to render $\LaTeX$ in chat, such as the start ChatJax  bookmarklet, which uses MathJax to render.
In the Mathematics chat, we often use the codecogs.com $\LaTeX$ Equation Editor: http://latex.codecogs.com/gif.latex?\sum_{k=1}^\infty\frac{1}{k^2}=\frac{\pi^2}{6} yields 

You have to replace spaces with %20 since spaces are not allowed in URLs (actually, it is probably best to escape all reserved characters in URLs, but other than spaces, most characters seem to be passed through by a number of common browsers).
A: I haven't seen the easiest answer: Facebook Messager! If you write a message of the form:
$$ equation here $$
The whole message has to be only the formula, if you want an equation as a part of a longer message you can use:
Some text \( equation here \) and more text.

A: I use overleaf and screenshare it while talking over zoom. Or I use a chrome extension "TeX for Gmail & Inbox"
