HTTrack - HTTrack is a free (GPL, libre/free software) and easy-to-use offline browser utility.
Beyond Compare - Beyond Compare allows you to compare files and folders.
WebCopy - Cyotek WebCopy is a free tool for copying full or partial websites locally onto your harddisk for offline viewing.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
SiteSucker - SiteSucker is a Macintosh application that automatically downloads Web sites from the Internet.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.