HTTrack
Open Source
HTTrack is a free (GPL, libre/free software) and easy-to-use offline browser utility.
UnMHT
Open Source
UnMHT adds MHT file reading/writing support to Firefox and SeaMonkey.
ScrapBook
Open Source
ScrapBook is an extension for the Mozilla Firefox web browser which adds enhanced scrapbooking, page saving, bookmarking, and notetaking functionality.
Compare Offline Explorer with alternatives
Compare HTTrack and Offline Explorer and decide which is most suitable for you.
Compare ScrapBook and Offline Explorer and decide which is most suitable for you.
Compare UnMHT and Offline Explorer and decide which is most suitable for you.