Simple list of free open-source software for Windows
Open Source Windows has a pretty neat list of free open-source software for Windows grouped into various categories. I would say it’s a good place to start for those who are searching for an open-source/free alternative to the software they are currently using as they have handpicked the cream of the crop for each category.
While on open-source, I came across this page which has a list of open-source games for Windows. Haven’t tried any of them though. ;)

What does “Open Source” mean? And is it alrite to use those free open source program for commercial purpose?
In simple terms, it means that the source code for a product which is being developed is freely accessible by anyone. You can get more info on it here: http://en.wikipedia.org/wiki/Open_source
It depends on the license which the open source software uses. Some licenses permit it, some might have restrictions while some totally outlaw the usage in commercial enviroments. You might have to check the terms of use for that, but personal/home usage will not be a problem at all.
[...] With USB thumb drive storage capacities skyrocketing in recent times, you can now have those applications you need stored in it, ready to be used right away with Portable Apps. The list of applications being made portable seems to be growing so there is a chance more applications will be added soon if the one you’re after isn’t listed. All portable applications are open source and seem to be updated fairly regularly. [...]