Most distributions (or flavors) of Linux are free.
http://www.linux.org/dist/ <-- Start here
Updates are released and distributed daily - Linux isn't 'owned' by a corporation that releases service packs every few months.
Once you have it setup, and you have a GUI (graphical user interface) setup, it's quite easy to use.
I would suggest you do a lot of reading before making the switch, just so you completely understand what you are getting into. Might wanna start here:
www.linux.org
- Justin