Koffice is open source office suite for Linux , Mac OS ,windows and haiku OS. It has quite a lot of components. Its release 2.1 based on kde 4 has now been the release candidate stage. Windows release will take sometime. They still say it is not stable but it is. I think its fear of failure.It contains these as counterparts of MS office:
kword as counterpart of MS word ( word processor),
Kspread as equal of excel (spreadsheet),
kpresenter much like power point (presentation),
kexi like MS access (database),
krita like MS paint ,
kivio like Visio (diagramming and flow charting),
Kplato like Microsoft Project ( project planning),
karbon for vector graphics.

It also have a mobile version now. MS 2007 filters will be available from Q2 2010 also.So it means 100% compatibility soon. It all features you need in a office suite and its default install is more powerfull than openoffice but latter has plethora of plugins.
Support is through user-base , irc channels and forums.