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


From Diablo 2 Wiki

Calculators can be used to calculate an exact approximation of what particular monsters will drop, how much damage various attacks will do, and other such interesting and valuable information.

The most popular and accurate calculator is ATMA, which you can read about and download here. In addition to making drop and damage calculations, ATMA allows players to view their single player characters through the application, and even trade items between them. Note that ATMA wasn't updated for patch 1.12.

There are other online calculators and skill planners of varying quality and accuracy, but beware of any other than ATMA that require an installation, or that promise to enable hacking or cheating or that works as trainers; countless players have lost characters or had their accounts hacked after using such third party programs.