Kiwix is a browser which allows you to open and browse compressed .zim files, which can contain all of the content from a website, so that content becomes portable and available offline. It was developed to support education in developing countries with limited or unreliable internet access, and the Kiwix catalogue includes a range of content from the Wikimedia Foundation, Project Gutenberg and a very big range of other sources.
I have the complete English Wikipedia 'maxi' version with photos (124GB), Wiktionary without pictures (9GB), and Gutenberg English Literature (16GB) on an external USB hard drive. There is also a server version of Kiwix available if you want to make .zim files available on your lan, user access being via web browser.
Kiwix website
Kiwix database catalogue
The Kiwix catalogue has a lot of content, but what if there is something you really want which isn't there? Kiwix has a tool for that on its website, called Zimit, which you can use to create a .zim database of any website you specify, which it will do on Kiwix servers and then allow you to download the finished file.
I used it to create an offline version of Pagat.com, which is a wiki-like website that has detailed rules for hundreds of traditional card, domino and other tile games. It has been around since 1995, but it's a one-man effort, the kind of Web 1.0 website which looks vulnerable to anything happening to its founder and owner, so I wanted a local copy. Of course I have made appropriate cash contributions to both Kiwix and Pagat to acknowledge the value I get from this.
In use Kiwix works like a simple browser, presenting the content in a very acceptable way that looks much the same as it would 'live' in a web browser. Internal Wikipedia links open in Kiwix and external links in a new window in your default web browser. Navigation isn't particularly fast and probably no quicker than the live Wikipedia site would be on my average ADSL broadband connection, which I think is because the .zim file remains compressed and the browser is therefore decompressing pages on the fly when you request them. Not really quick, but OK.
The main downside is that the files you store locally are fixed, and updating them is a manual task. There is a script available to check for updates, but I haven't tried that. In practice, something like Wikipedia changes all the time, but across the whole encyclopedia it doesn't change so much from one year to the next that I really care about it. I can get a new download from the catalogue any time I want to.
This shouldn't need saying, but Kiwix and Zimit are only free at the point of use. They are developed and maintained by a not-for-profit foundation and they have some sponsors and a 'pro' product which is paid for, and they sell ready built hardware hotspots, but if you use the free version it's important to give them some money via the Donate button on the website. After all, most of the 2 or 3 people likely to read this are not the intended beneficiaries in developing countries.
Email: kdan01 [at] posteo [dot] net