HTTrack is a free (GPL, libre/free software) and easy-to-use offline browser utility.
It allows you to download a World Wide Web site from the Internet 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 is the Windows 2000/XP/Vista/Seven release of HTTrack, and WebHTTrack the Linux/Unix/BSD release. See the download page.