For most uses, KOffice works well (Windows or Linux); GNOME Office (AbiWord, Gnumeric, GIMP, etc.) also work well, but I don't think there is a Powerpoint analog.
For Windows computers, the best office package I've found yet is Ashampoo Office -- it's not free, but it's cheap and frequently discounted. Ashampoo being a German software developer, they don't get many writeups in the US, but the few reviews that exist are generally quite positive, and my own experience with their software is extremely positive. I prefer Ashampoo Office to Microsoft Office -- it has nearly all the functions (including Track Changes) yet is much simpler to use, has a cleaner interface, and takes up much less space on whatever hard drive it is installed on.