Journeyman (to 150)
Finbus Geargrind (Duskwood)
Lilliam Sparkspindle (Stormwind)
Trixie Quikswitch (Ironforge)

Expert (to 225)
Springspindle Fizzlegear (Ironforge)

Artisan (to 300)
Buzzek Bracketswing (Tanaris) [Recipes]
Gnome Engineering Quest
Goblin Engineering Quest

Master (to 375)
Lebowski (Hellfire Peninsula)
K. Lee Smallfry (Zangarmarsh)

Horde:

Apprentice (to 75)
Graham Van Talen (Undercity)
Mukdrak (Durotar)
Thund (Orgrimmar)
Tinkerwiz (The Barrens)
Twizwick Sprocketgrind (Mulgore)

Journeyman (to 150)
Franklin Lloyd (Undercity)
Nogg (Orgrimmar)

Expert (to 225)
Roxxik (Orgrimmar)

Artisan (to 300)
Buzzek Bracketswing (Tanaris) [Recipes]
Gnome Engineering Quest
Goblin Engineering Quest

Master (to 375)
Zebig (Hellfire Peninsula)
Mack Diver (Zangarmarsh)