7zip is a good suggestion, as it is superior in many ways to winrar, and you end up supporting a better software model by using 7zip.
Also while games on a computer are typically software, I don't think that's what was meant, they are hardly pieces of 'softwares you must have', but just to point at a good example of a physics game, 'Phun' is lots of fun.
I mentioned wolframalpha.com in a previous post as a powerful calculator, but if you need something offline then
Wxmaxima or
Python are useful to have, and both free (technically python is a programming language etc but it has built in arbitrary sized floats and an easy to use maths library which makes it a good calculator as well.)