HTTrack is a popular but free and easy-to-use software which can be used to download complete website for offline browsing.
HTTrack can download a complete website to a local directory, building recursively all directories, getting HTML, images, and other files from the server to your computer. HTTrack arranges the original site's relative link-structure. Simply open a page of the "mirrored" website in your browser, and you can browse the site from link to link, as if you were viewing it online. HTTrack can also update an existing mirrored site, and resume interrupted downloads. HTTrack is fully configurable, and has an integrated help system.
WinHTTrack works on Windows 9x/XP/Vista/Seven as well as on Linux/Unix/BSD systems.