The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section

Category:Hellfire

Category page

See the Hellfire page for basic details about the expansion pack, and instructions on how to enable its hidden features.

Almost all features in Diablo are found in Hellfire, but Hellfire added quite a few features, including 3 new character classes, two dungeon areas, oils, runes, numerous new unique items, and much more.