eReader software allows you to read secure (encrypted) eBooks on PalmOS Handheld or Smartphone, Windows Mobile PocketPC (Professional), Windows Mobile Smartphone (Standard), Symbian s60 and UIQ, Windows PCs and Laptops and Apple Macintosh. The software is free and can be downloaded here.

http://www.fictionwise.com/help/ereader-download.htm

Adobe's "Digital Editions" Version 1.5 now has versions for both Windows and Mac, with a Linux version still awaited. Like its predecessor the Adobe eBook Reader, this is a much smaller software than the Adobe Reader and is specific for e-books. The download is 4.3MB for the Windows version. Digital Editions software will now read not only PDFs but also the new, fluid ".epub" file specification. Useful features include the abilities to change font size, add bookmarks, make text notes and highlight passages, while SWF multimedia files are supported too. Digital Editions also offers a library mode and a bookshelves feature.
"Digital Editions " is currently offered in English, French, and German versions and can display content in a wide range of languages. Please note the minimum system requirements.

Adobe Reader
Go to this URL to find the most recent Adobe Reader version compatible with your desktop or laptop operating system. The Adobe Reader is a larger, fully-featured software for general use with PDF files.
http://www.adobe.com/products/acrobat/readstep2.html
General Adobe Version Notes:
*The latest Adobe Reader version is the Adobe Reader 9 (released July 2008).To use Adobe Reader 9 on a Windows OS PC you need Windows XP or Windows Vista.  If you go to this Adobe page and select your OS and version, your system will be scanned and you will be informed of the highest Adobe Reader version you can update to. If your OS predates those shown, you should retain your existing version. Specifically, If you are running Windows ME or 2000 ignore any upgrading hints the Reader may give, and do not go beyond Adobe Reader 7.  If your Windows is 95, 98, or 98SE it is unwise to go beyond Adobe Reader 6. Note that although dire warnings may flash up, most PDFs will still display successfully with Adobe Reader 6 or above.
* New features of Adobe Reader 9. Fully supports the newest PDF file versions. Can interact directly with Adobe Flash software. There are useful improvements to search abilities too. This release is available for Windows and Mac operating systems only so far, in English, French, German, and Japanese language versions.  Further OS and language support is expected later.
* Since the full release of Digital Editions (see above) Adobe Reader Versions 8 and above will not themselves read DRM-protected PDF e-books, although Digital Editions can be launched from within Adobe Reader 8 and above. However if you want to continue reading DRM-protected e-books & don't want to install the Digital Editions software, stick with earlier versions of the Adobe Reader.

Microsoft Reader for Desktop and Laptop PC
Free software, available in English, French,German, Italian or Spanish versions. If you have an older version already installed, be sure to uninstall it first.
http://www.microsoft.com/reader/downloads/pc.asp

FBReader is a free e-book reader for many Linux-based portable devices and Linux and Windows XP desktop computers. It supports the following file formats: CHM, .fb2, HTML, mobipocket (non-DRM only) OeB, OpenReader, Palmdoc, Plain text, plucker, RTF, TCR zTxt. The various versions were still in Beta format at mid 2008, so please consider that factor before installing.
http://www.fbreader.org/downloads.php