I haven't heard about any single group handing out any financial support to open source projects.
There are often differences of opinion between projects, which is what spawns the offshoot project.
nope, simplyMephis isn't based on Ubuntu, it has been around long before Ubuntu was.
a single donation that would have the broadest support for open source projects would be to donate to sourceforge, gnu.org or savannah.org [ last two are tied together, savannah is the gnu source repository and project site that compares to sourceforge. ]
Sourceforge doesn't actually need financial donations, VASoftware, the owners, sell licenses for using the sourceforge collaboration software at very high rates, which pays for the development and funds sourceforge. they do ask for and accept mirrors, if you have an OC3 internet connection.
You will most likely be best to give donations to the project(s) you want to support directly.
editing to add:
OK, there is a group that does fund projects, I just found a link to them on php.net..
http://www.linuxfund.org/
I was mistaken in thinking there was no such group