There is no web user who is not aware of Wikipedia.We all love it and Google loves it too, As it is the top trusted site by this search engine giant. If you got enough space to spare for this wonderful encyclopedia,
You can download it for free
you can download the whole Wikipedia in your favorite language, It can be downloaded directly or by using Bittorrent (18.2 GB). This 18.2 GB website copy will be downloaded in .ZIM file format. You can use Kiwix software to access that .ZIM file.
For those who don't want to spare 18 GB of your free space, You can download Mini-Size Wikipedia version of 3.74 GB containing 45,000 top articles. For this encyclopedia, I think we can spare that much space. We can even move that file in to our Smartphones as there is Kiwix app available for android.
Wikipedia 18.7 GB file
Wikipedia 3.7 GB file