HTTrack is a free (GPL, libre/free software) and easy-to-use offline
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.