Online White Board - a tool for instant communication. Best for Skype disscusstions.
(support page/bugs reporting)
To keep track of your arXiv usage.
If you want some formulas
on your web page you can use this:
Free simple program to use LaTeX in PowerPoint (for Windows).
Convert your mathematica notebook into a basic LaTeX editor.
Sorts references in LaTeX files (if you don't like bibtex).
Main page of thd