Mods are basically patches made by users that allow you to play single player
with new features then the game originally intended. This means new
expanding possibilities for graphics, monsters, weapons, spells, stash &
inventory size, and so much more. Some of the best mods for games can
alter the play so much, it may feel like a whole new game.
The best external sources you can find updated lists of mods for D2:
PlanetDiablo DB - Great listing (large) and sorts by version. Also contains direct website link for updated versions if not yet posted.
Mod DB - Another favorite moding source
of ours. However some of the mods may have been forgotten
about/discontinued and are still on the list.
EG Diablo 2 Topsites - Though this site isn't
explicitly for mods, you'll find a few sites dedicated to individual mods and
other sites with simular mod listings as the ones above. You can use the
search feature to find some mods.
Top 200 Sites - An older version of the above
site (but contains more links)...Just do a page search for "mod".