The best of D&D

[Pinotage]

3e and 3.5e have been out for quite some time now, and there's a host of supplemental material out there. What I'm interesting in knowing via collective brain dump is what are the best non-core game mechanics/rules that enhance the core system (PHB, MM, DMG) the most. This could be anything from a feat (e.g. Sudden Metamagic) to a new magic system (Elements of Magic) or even something as small as an alternative use of a skill.

What are your must have rules/game mechanics and from what sources do they come? I have no hope of owning, or even reading, a fraction of the game material out there, so I'm hoping to hear what you think is the best gaming material (not product, just which part of any given product, unless the whole product is fantastic) out there that no player/DM should be without. What will enhance any core game even further?

Pinotage

 

[DMH]

I will keep it short:

Best craft, profession and appraise rules- Enchiridion of Treasures and Objects D'Art (ST Cooley)

Best paragon classes and leveled templates- Shaping the Self (Genjitsu)

Best poison rules- Toxicant (Mad Kaiser)

Best businesses write up without NPC stats- Kaiser's Bazaar A, B and C (MK)

Best alchemy rules- Alchemy and Herbalists (Bastion)

Best law enforcement rules- Crime and Punishment (Atlas)

Best political rules- Dynasties and Demagogues (Atlas)

Best cold environment- Frost and Fur (Monkey God / Highmoon)

Best weapons guide- Arms and Armor 3.5

Best template book- Book of Templates (Deluxe Edition) (Silverthorne)

Best combat book- Torn Asunder (Bastion)

Funniest spell books- Blasphemous Secrets and Forbidden Lore (Plain Brown Wrapper)

 

[Particle_Man]

Best new base class: Warlock (Complete Arcane)

Best new feat: Practiced Spellcaster (Complete Arcane/Complete Divine)