Been testing it out today and I'm quite impressed. Very functional, haven't found anything I can't do on it that I can in Microsoft office - although my work doesn't require very complicated actions. So far I've found it 100% compatible using excel documents but the downside is that it isn't compatible the other way round. It's a freeware program available for Windows and OX.
More info from http://www.openoffice.org
More info from http://www.openoffice.org