Costuming & props:

The Steampunk Emporium

Clockwork Couture

Fashionable Hats

Pyramid Collection

Wicked Clothes

Lip Service

Museum Replicas

Steampunk Threads

Retro Scope Fashions

Gallery Serpentine

The Village Hat Shop

Maskworld

Gloves-Online

(and check your local thrift shop, eBay and Etsy for vintage clothing)

Hardware (for DIY projects):

Grand Brass Lamp Parts

Buy Railings

Bolt Depot

K&S Engineering

Ridout Plastics

Ramsey Electronics

Lapel Pin Planet

HHH Enterprises

LCM Lab

Websites:

Steampunk Tendencies

The Steampunk Empire

Steampunk Writers & Artists Guild

Steampunk Chronicle

Brass Goggles

Aether Emporium

The Steampunk Librarian

Wings of Steam

The Steampunk Tribune

Steampunk Lab

The Steampunk Workshop

Steampunk Bitz

The Steampunk Home

Steampunk Pics

Dr. Grymm Laboratories

Airship Ambassador

Conventions:

Steampunk World's Fair

TeslaCon

Chrononaut

Anachro Con

Anamoly Con

The Florida Steampunk Exhibition   

Victoria Steam Exposition

Gaslight Gathering

CMRI Steampunk Meetups

Waltz on the Wye

Clockwork Alchemy

Steam Revolution

Dragon Con

The Asylum

Octopodicon

SteamCon IV

Organizations:

The Steampunk Empire

Steampunk Writers & Artists Guild

The Steampunk Club

Steampunk-Club

San Diego Steampunk

Steampunk (on FanPop)

History:

Steampunk history on Wikipedia

Steampunk Scholar

Tor - Steampunk and History

Jules Verne

Jules Verne (Canadian Website)

H.G. Wells

The Wild Wild West (TV Series)

Artwork:

Fine Art of America: Steampunk

Dark Roasted Blend

6D6 Steampunk Art

Running Like Clockwork

Inspire Monkey: Showcase

Museum: Art of Steampunk

Music:

Abney Park

The Clockwork Dolls

Steampunk music on Amazon

Long Stride Lizzy

Frenchy and the Punk

Publishers:

Tor.com

Penguin Books

Steampunk Tales

Steampunk Magazine

Tachyon Publications

Smashwords (indie publishing site)

Amazon Kindle Direct (indie pub.)