The first software I see as a worthy alternative is called Abiword. It is today's fast and most powerful word-processor. It makes it very easy while writing a large letter and/or document due to it's fast and simple word processor engine. Open source, compatible multiplatform and easy integration with Microsoft Office is all there with Abiword. The main reason that this can be considered is due to its reduced footprint on your hard disk and RAM.
Other nearly indispensable software nowadays that one can use in order to create and modify calculation sheets (the equivalent of Excel of Microsoft), is called Gnumeric. Gnumeric is again Open source, multiplatform, fast and light.
Did you feel that we lack one good alternative to Outlook (which isn't there with the Openoffice.org Office Suite)? Obviously not, we have Evolution. It's again multiplatform and also open source.
And at last, only for the Linux users, an optimal software in order to create and to modify Presentations is Kpresenter. Obviously open source and can also edit and display Microsoft Powerpoint Slides.