Most of the good ones you need to pay unfortunately ... I have yet to find any free applications that can open (even for just read only purpose).

I believe the best apps out there (which you have to pay) are those that have already been mentioned in the forum:

1. Write, Pages and Numbers (by Apple) - USD9.99 per app
2. Docs To Go
3. QuickOffice

and there was another one .. can't recall the name now