Here is a great list of free alternative office suite that able replace Microsoft Office suite without any cost and free for use. As Microsoft Office has been the de facto standard in office software suites for many years. But the Microsoft office suite software costs a lot of fortune and some of us might not affordable for the office suite. However, there are available alternatives to Microsoft Office, and a number of them are totally free.
List of Free Alternative Microsoft Office Suite :

OxygenOffice Professional is enhancement or a repackaged version of OpenOffice.org. The OxygenOffice suite includes a wordprocessor, presentation software, spreadsheet, database, drawing software, web editor, equation editor and so on. However, OxygeOffice professional is more advances compare to OpenOffice.org due to it support run Visual Basic for Application (VBA) macros in Calc (for testing) , Improved Calc HTML export, enhanced performance and color palette. Plus more, OxygenOffice suite contain more than 3200 graphic clip art and photo, over 90 free fonts , several professional templates and additional tools like OOoWikiPedia to enhance the office suite productivity.
Meanwhile, OxygenOffice Professional tracks the OpenOffice.org releases fairly closely which you should be able to get a version that includes the current version of OpenOffice.org. In addtion, OxygenOffice Professional suite workable on Windows and Linux platform.
Download OxygenOffice Professional Suite

IBM Lotus Symphony , a free office suite that build by IBM based on earlier version of OpenOffice.org. It includes mostly all openoffice features and functionality. However, IBM Lotus Symphony provides three office productivity module only like wordprocessor (Lotus Symphony Documents), presentation software (Lotus Symphony Presentations) and spreadsheet (Lotus Symphony Spreadsheet) only. But IBM Lotus Symphony able to open number of office file formats, including the Open Document Format (ODF), Microsoft file formats (eg, .doc, .ppt, .xls, .dot, etc), Lotus SmartSuite formats, etc. This Lotus office suite work on Windows XP, Windows Vista and Linux.
OpenOffice.org

Openoffice.org office suite is the most famous among other free office suite. It’s build based on commercial Star office suite and provides similar features and functionality like Microsoft office suite. Plus more, i like Openoffice office suite because it support export your document file in the PDF format. Meanwhile, it offers a complete basic set of office suite includes word processor, spreadsheet, presentation manager, and drawing program, with a user interface and feature set similar to other office suites. Plus more, Openoffice.org office suiteis available on numerous platforms, including Windows, Linux, Mac OS X and Solaris
Download OpenOffice.org Office Suite
Sumber dari Techmixer

Kayaknya OpenOffice udah lumayan memadai sih kalau hanya untuk day-to-day use.
yah … open ofice mang mantapp …
[...] 27 September 2008 oleh Adi Gunawan Bagi yang sudah membaca posting sebelumnya soal alternatif Microsoft Office, maka berikut ini adalah alternatif MS Office yang paling identik, alias kloning karena dari [...]
mo tanya,… Lotus Symphony Documents udah bisa nambahin rumus ngga di dokumen yg kta buat? btw anda sndiri pake apa? ada kesan dari berbagai jenis Office ALternative ? jelasin jga dong advantages/diss nya
thanks
@ #4
Belum pernah pake Lotus Symphony
Kalau yang pernah dan masih saya pakai di Ubuntu ya OpenOffice, tapi buat platform Windows mungkin yang ada di http://alexgunawan.wordpress.com/2008/09/27/kingsoft-office-alternatif-dan-kloning-microsoft-office/ bisa jadi alternatif terbaik buat menggantikan MS Office karena sudah tidak perlu belajar lagi
mana program yg filenya bisa dibuka microsft office?