OpenOffice has been for long time a free alternative to MS Office.
On the Mac it needs to install X11 - which can be a bit annoying. There is though an interesting alternative: NeoOffice. It is an OpenOffice independent branch dedicated to the Mac and based on Aqua. It also opens the new MS formats (e.g. ppx) - OpenOffice does not yet! It also has an interface to Databases (can access a database through JDBC).
NoMu is a nice add on that installs a menu in the finder for launching NeoOffice in a different mode (e.g. as presentation or spreadsheet tool).
Alternatively, you can launch it from the shell as
/Applications/NeoOffice.app/Contents/MacOS/soffice.bin -impress file.ppt
/Applications/NeoOffice.app/Contents/MacOS/soffice.bin -calc file.xls
Here are some resources: