Below you will find easy to find items deemed either necessary or extremely important for each profession, although some may not be "essential" for the development of your character.

General

These items are essential to any character, regardless of profession.

Adventurer Items

Pilgrim Seeker Armor

Agent Items

Bureaucrat Items

Briefcase of Holding - Portable bank terminal. Made via tradeskill
Miyashiro Superior Enhanced Coffee Machine

Doctor Items

Treatment and Pharmacy Library Auno
Temporal Chalice (level 25-60) from Nematet in Temple of the Three Winds
Holy Book of the Immortal (level 25-60) from Aztur the Immortal in Temple of the Three Winds
Teachings of the Immortal One (level 125+) from third floor of The Inner Sanctum

Enforcer Items

Library of Foul Language Cyborgs
Aggression Multiplier (Jealousy Augmented) Steps of Madness + Shop item

Pilgrim Protector Armor

Engineer Items

Trimmer - Positive Aggressive-Defensive - This trimmer increases the aggressiveness of a robot pet (including Bureaucrat robots) so they attack faster and draw more aggression from monsters. The effects of the trimmer last until you zone into a new area. It can be store bought up to ql 100
Trimmer - Divert Energy to Offense - This trimmer out right adds to the damage of a robotic pet (including Bureaucrat robots) so they deal more damage every hit. The effects of the trimmer last for 5 minutes or until you zone into a new area. This trimmer can be store bought up to ql 200
Trimmer - Increase Aggressiveness (Wiki)- This trimmer also increases the aggressiveness of a robotic pet (including Bureaucrat robots) so they will more likely keep the attention of monsters. The effects of the trimmer last until you zone into a new area. This trimmer must be tradeskilled
Repairman's Hat - Trade skill - Procedure
Construction Sleeves - Subway