openoffice.org
If you want free, this is the way to go. There is also the Google Docs as stated above, which are becoming quite popular. But Open Office will open and work with the Microsoft Office file types just fine.
If and only if there are no macros attendant to whatever file you might be opening. If there are VBA macros they absolutely positively will not work with Open Office. OO uses a far different, and if you were fetched up with VBA incomprehensible, object model than VBA.
Open Office always puts one in mind of Dr. Johnson's observation on a dancing dog: "It's not that it does it well, it's that it does it at all."