I'm just wandering is there some magento package that can be downloaded, without admin and all ecommerce modules, just a clean framework ready to develop my own modules from scratch?
Have a look at this. Probably the closest thing to what you are looking for.
Have a look at Magento Lite, that's what you are looking for;
And this is the lite version of Magento so far we get freely :) for general purpose.
Otherwise you will have to disable more modules from it based on your need