You might want to check out Apple's iWorks. It's 99 Dollars, but you can download and try it for free. one of the programs in iWorks is Pages. It's a word processor and basic layout application. it's very easy to use. And it includes templates for making posters.
Open Office is a free Alternative to word and pages. You can use it to word process, and It can open, and save word files.
http://www.openoffice.org/
Pixel Mator, is an alternative to photo shop. You can use it to make posters or edit photos. You can download it for free, but it costs 50 dollars,
http://www.pixelmator.com/
A free Alternative to photoshop would be Gimp. It's kind of hard to use in comparison to pages and pixel mator. BUt it can be used to make posters. It can be downloaded at
http://www.gimp.org/ .