Openoffice is a complete free office suite which you can use for word processing, spreadsheets and more. I don't really use it very often, because I use LaTeX for writing documents and R for doing my math. However, if you're looking for free software that has the same functionalities as MS Office, Openoffice is a very good option for you. Check it out!
Gnumeric is a nice spreadsheet application. Faster than the one you find in Openoffice.