What type of posters? Would you be using photos, illustrations, or both? There is so much that can go into something like a poster.
I would say InDesign is probably your best bet when it comes to layout, typesetting, and print. You can easily import pictures and all that fun stuff.
When I make a poster for a client, I usually end up using a mix of Photoshop, Illustrator, and InDesign. Although the bundle is not the cheapest you are going to find, it will handle just about anything you are trying to do. 😀
I can't help you with free programs as I have not used freebies in a long time 🙁
If money is an object, than you can buy InDesign as a standalone program and you will be able to accomplish your basic poster design within it.