Or how about the original product from Sun Microsystems:
StarOffice - also free !
It's pretty much platform independant, although Mac users might like a read of this:
http://porting.openoffice.org/mac
I have actually used StarOffice and must say that I like it (actually, quite a lot !). Indeed if you order the CD (only a few quid) you can actually install the equivalent of MS-Exchange but for a fraction of the cost.