Here are several FREE software Applications that I have used.
Kindle for PC - Download the software and you can read mobi file (Kindle) e-books on your PC/iphone/Mac/Blackberry/Android/iPad/Windows7phone.
Nook for PC - Download the software and you can read E-pub (Nook) e-books on your PC.
Adobe Digital Editions - for .pdf file format. The Adobe Digital Editions allows the reader a more 'book' type experience than the old Adobe Reader.
Calibre - E-book reader. It also converts one type of e-book into another. Example: You buy dozens of e-books from Amazon (MOBI format) then you get a Nook (E-pub format) for birthday... Ouch! But don't worry, Calibre will convert e-book formats and it is very user friendly.
OpenOffice.org - If you don't use MS Word, try this free program. www.cnet.com for the download and getting Go-00 first (free software). Go-00 will speed up OpenOffice.