HTTrack is one impressive product: it's easy to use, has an excellent user interface, offers every feature you could want, while being blindingly fast and free of any adware. If you like to download web sites so that you can "browse them offline", this is the product to get. HTTrack is compatible with all windows versions, and is GPL licensed freeware. WinHTTrack is the Windows release of HTTrack, and WebHTTrack is the Linux release. There are no official binaries supplied for the Mac OS, but the FAQ list details on how they can be compiled from the original source files.
Alternatively, there is the free edition of PageNest, which allows you to download and categorize websites, as well as browse them within the programs own viewer. PageNest can also show what new or updated content has added to an archived website. The Pro version of PageNest, among other things, adds the ability to schedule auto-updates to archived sites.
The third program in this category, Local Website Archive (Lite Edition), only allows you to download web pages and other documents on the web. However, it as good integration with Opera, Firefox and Internet Explorer, allowing you to capture a web pages URL directly from the browser. Unlike HTTrack or PageNest, this program has a built in search facility, which identifies the pages that a word or phrase appears in, and highlights the words in the document. It also has the useful ability to zip up a stored page, so it can be emailed.
HTTrack
Website: http://www.httrack.com/
Download page: http://www.httrack.com/page/2/en/index.html
Author: Xavier Roche & other contributors
Current version: 3.42
Version date: 19th March, 2008
License: Free - GPL Open Source
Download File size: 3.3MB
Operating Systems Supported: Windows 95 – Vista, Mac, and various Linux distributions
64 bit version available: Untested
Portable version available: No, but zip file version available
Non-English languages supported: Many; Available via downloadable language packs
PageNest
Website: http://www.pagenest.com/
Download page: http://www.pagenest.com/download.html
Author: Mike Sutton
Current version: 3.04
Version date: December 2008
License: Free – For personal use only
Download File size: 1.74MB
Operating Systems Supported: Windows 98 - Vista
64 bit version available: Untested
Portable version available: No
Non-English languages supported: None
Other relevant information: Commercial version available
Local Website Archive
Website: http://www.aignes.com/lwa.htm
Download page: http://www.aignes.com/lwa.htm
Author: Aignesberger Software GmbH
Current version: 2.1.0
Version date: 26th March, 2008
License: Free – for personal use
Download File size: 1.5 MB
Operating Systems Supported: Windows 95 - Vista
64 bit version available: Untested
Portable version available: Yes, installer has USB option
Non-English languages supported: Possibly Russian
Other relevant information: Commercial version available