Idiosyncratic List of Suppliers
arbitrarily selected by Michael Schippling
-- if you have companies to add send me something --

Commercial and Industrial Suppliers

These guys pretty much have everything if you know how to search for it. They are generally hobbyist friendly for small quantities, but not the cheapest.

Electronics

DigiKey -- Goto for wide selection.
Newark -- Good search options.
Mouser -- Haven't used but reputable.
Arrow -- Haven't used but reputable.

Mechanical, Hardware, and Tools

Manhattan Supply -- about every tool ever made.
McMaster-Carr -- just about every nut and bolt ever.
Grainger -- some things McMaster missed, +$$$.
VXB Bearings -- inexpensive bearings.

Electronic and Misc Surplus

Don't always have everything, but it will be cheaper if they do and you might find some other interesting stuff along the way.

JameCo -- old standby.
Electronics Goldmine -- good selection of surplus stuff.
MPJA -- plus cheap knockoff Arduinos.

Hobbyist Supplies and Assemblies

Arduinos and shields, RasPi, robot parts, etc.

AdaFruit -- The Lady Ada's operation.
SparkFun -- Boulder CO's answer.
Servo City -- robot parts galore.

Raw Materials

Small quantities of metal and plastic delivered to your door.

Online Metals -- metal and plastic.
Metals Depot -- metal.
Delvies Plastics -- plastics.

Computer Components

Usually good prices on standard PC parts.

NewEgg

Printed Circuit Card Manufacturing

There are many others. These guys are not the cheapest but they give you schematic and layout software that is (almost) easy to use and deliver prototypes in 3 days.

Express PCB

Idiosyncratic List of Suppliers arbitrarily selected by Michael Schippling -- if you have companies to add send me something --