Online physics collaboration tools I.e. online discussion with your friends. A forum is probably too overkill in this case.
Yet so far nothing can beat direct communication.
Important feature: the ability to archive discussions. We don't want to retell our story to people who just missed the "conference" (let the newcomers dig through the archives by themselves).
 A: I should mention the wonderful userscripts written by Valery Alexeev which utilize mathjax and allow you to render latex on any webpage, even if it does not support such rendering internally. Currently supported webpages include arXiv and gmail, though it should be easy to add your own to the list.
Used in combination with mathim.com and Tiddlywiki we have a powerful platform for online tex-enabled collaborations.
A: 
http://www.mathim.com/

Latex based chatroom partially fulfills the requirement . The best scenario I can imagine is a mathjax/latex based wiki/website with an embedded $\LaTeX$ chat like M$\alpha$thIM. 
A: convore: $\frac1{\sqrt3}(|IRC\rangle + |mailing list\rangle + |chat\rangle)$
No built-in latex support yet, but this can be remedied by installing the display latex userscript (thank you Deepak Vaid).
A nice complement to convore: online whiteboard (e.g. scriblink, flockdraw).
A: alternative to tiddlywiki: instiki
a simple wiki with latex support out of the box.
I haven't really explored it yet, so please share your experience.
A: Rather overkill, but you may want to try MediaWiki
Requires: php/mysql
Install time:5  mins not counting download.
For math, there is an incredibly long procedure to set up the partially built-in png math renderer.
Alternatively, you can just install the mathjax extension. Quick process not including downloads.
Also, the LiquidThreads extension makes discussions easier and organized.
I'll post some links later.
