Check http://code.google.com/p/powerpointtools for up-to-date information.
The LanguagePainter allows you to write slides in a language different from your current keyboard layout without PowerPoint messing the language settings up all the time.
This addin provides better support for formulas in PowerPoint. It supports both inline formulas and equations:
It supports rendering using LaTeX through webservices (currently it only supports the one from http://www.codecogs.com) and MiKTeX. The latter has some neat features like resolution adaptation and baseline alignment which the webservice lacks.
There are still some code clean-ups, small bugfixes etc that I should work on, but I don't plan on selling this addin, so I don't really care if it's still somewhat work in progress. I'm going to continue working on it when I have to use PowerPoint again.
You can download the tools out at http://code.google.com/p/powerpointtools/.
I'm not really done with the project but I think it's time nonetheless to talk about the project itself. Initially I looked into using TexPoint for a presentation I had to prepare, but it wasn't very user-friendly in my opinion and I also didn't really want to install MiKTeX again, so the idea was born to use a web service (from codecogs.com or wordpress.com) to render the equations and embed the formulas as pictures. A few tests later I knew that it was feasible, so I started to work on the project.
The bulk of the code was written in 4 days during my winter vacation in 2008/2009 and again in spring (May) 2010.
This warrants the question what took me so long to upload it and/or write about it. The main reason why it was delayed by such a long time is that I wanted to polish it some more and make sure it really has a well-rounded feature set. In retrospect I shouldn't have added support for non-inline equations because people will hardly use them and the way you edit them is kind of crazy (I create a text shape in the current slide to let the user change the equation) and I reverted lots of the later changes because I wasn't sure which direction to take UI-wise.
The user interface is mostly okay in my opinion but the ribbon might need a redesign to streamline it some more and also maybe come up with new things users would want to do. For that I'll probably wait until I have to use it again (probably in a few weeks' time) and then take notes about what to improve.
The design of the code is somewhat messy when you look at it, but that's because it grew out of a experimental project to see if it was possible to embed LaTeX code the way I wanted, and most of the functionality ended up in one big file/class (LaTeXTool.cs) because everything is centered around inline formulas.