How do I let my friends use my Python programs? They don't have python installed, but they can use command line programs. I don't have the skill to write GUI.
Let, say I am writing some calculus calculator, with a lot of custom modules and files. How do I share it?
You could use something like py2exe to convert your Python program into an executable.
You have the options presented thus far: Portable Python and Py2Exe. Either can be pretty good.
My suggestion: encourage your friends to install Python! As you know, it's free, and simple to install and download. If they want your application bad enough, installing Python will be a no-brainer.
As musicfreak said, it would be helpful to know what OS your friends are using. One reason being that Mac OS/X and many versions of Linux come with Python pre-installed.
A less general, but lightweight and simple way of putting many Python files into 1 or 2 files (Python programs) is Fredrik Lundh's squeeze program. When you "squeeze" a bunch of Python programs and modules, you can often produce a single Python program. People still need Python to run it (but it's included in most Unix distributions, including Mac OS X), but you can easily distribute it, as your program and modules are all bunched up in a single file.