Google Docs. Free, totally compatible with Word (they also have their own versions of Excel and Powerpoint), and the added bonus of online access. They're stored in the "cloud" and you can access them from every computer you use, PC or Mac. I'm a h.s. teacher and I also use Google Docs for student research. Students can share their documents with other students for group work, or their teachers for editing and comments. VERY easy to use!