The Summoning tree provides multiple options for minions the Poison and Bone tree allows powerful poison damage, magic-based bone damage and defensive spells, and the all-powerful Corpse Explosion that brings area-of-effect fire and physical damage and the Curses tree allows complementary spells that enhance damage or provide major crowd control. The Necromancer probably has the most variability of possible builds among the Diablo 2 character classes.
