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"Not the best, but still a good deal" This is not an exhaustive list, but these are my favorite surplus locations. Check in for specials, sales, and limited-quantity items.
General Surplus parts and components
Dan's Small Parts and Kits A wide variety of transistors, diodes, inductors, caps, crystals, discreet RF devices, standoffs, connectors etc.
Evil Mad Scientist Laboratories lots of fun kits, cool "BulbDial" clock, awesome 12" and 2.3" 7 segment displays, LEDs, ICs, Servos, Stepper motors, tools, Proto and bread board stuff. Ribbon, Servo and USB cables.
Jameco Electronics Lots of parts and you can order a paper catalog. they also have grab bags of lots of commonly used parts(LEDs, resistors, capacitors, 7 segment displays, stand offs and hardware, etc.)
eCrater Lots of random stuff on the rest of the website.
Robot Shop Lots of Robotics parts, servos, kits.
All Electronics Corp. Thousands of electronic and electro-mechanical parts and supplies at discount prices. Fast, reliable service. Most orders shipped within 24 hours.
The Electronic Goldmine Discount electronic components - check out their online special offers.
BGMicro Wow they sell random things, for really cheap - check out the specials!
Alltronics These guys always have something interesting for sale at low cost.
dipmicro electronics Huge and varied selection of ICs and sensors, hobby components, PCB prototyping and surplus FR-4, soldering stuff.
Electronix Express Some great deals on assorted stuff, proto boards, resitor kits ect..
MECI Liquidation Outlet Lots of electronic (and other) surplus.
Electronic Surplus Inc. Misc parts
Cascade Surplus Lots of goodies.(As of Jan. 2012 Cascade no longer does retail or web sales.)
Web-Tronics Misc parts and such
Hosfelt Electronics Misc parts, but has Burr Brown chips still in stock!
Surplus Sales of Nebraska "One of the largest selections of new and surplus electronic parts in the country right at your fingertips. Browse our website with over 20,000 individual items and photos. Hard to find RF and transmitting components as well as a huge selection of Collins parts."
Minute Man Electronics A great website especially for prototyping chemicals/supplies, but they also sell components. The $20 spending minimum is not hard to reach (trust me there). I was able to get 2lbs FECl3, 1/2 gallon of Tinnit, several hundred feet of stranded wire, and battery holders to make more MintyBoosts than I'll ever use shipped (quickly) for under $60. Email them for quantity price breaks or to pay with Paypal.
American Science & Surplus Not electronics, but a lot of fun sciency things. Formerly JerryCo - Lots of hobby items, good prices.
Excess Solutions Excess, surplus, obsolete items.
Herbach & Rademan Products for science and industry, mostly motor related.
Brigar Electronics Inc. website no longer active
Fair Radio Sales Company Inc. Mostly vintage and military parts.
Tanner Electronics Located just outside Dallas they are worth the trip if you are ever in town. They stock everything from enclosures to connecters, LEDs, switches, resisters, etc. I believe they will also ship so it's worth a call.
West Florida Components Online source for electronic components and supplies including current and obsolete parts (IC's, LEDS, capacitors, diodes, transistors, resistors, fans, ferrites, teflon tubing, etc) project boxes, perf boards, and more.
HSC Electronic Supply Really cool California surplus electronics retailer. The web store is a bit wonky, but walking around their huge storefront in Santa Clara is a highlight of my trips to the Bay Area.
Electronique Addison Electronics Surplus store in Montreal, Quebec, Canada. Lots of cheap parts. Awkward webstore, but good for folks in Canada who want to ship Canada Post and avoid duty, brokerage etc.
Wholesale Parts Suppliers
Parts Express Mostly audio/video now, but has valve/tube parts.
Antique Electronics Supply or AES Mostly Tube/Valve related
CE Distribution, the "Wholesale" arm of AES. You have to apply for an account for your business, but it's worth it as parts are much cheaper. 10 dollar fee for orders under $50
Super Bright LEDs Special led lighting configuration, reasonable prices.
Ramsey Electronics Products and kit items.
MWK Lasers by Mike Kenny Long time reputable dealer of low cost laser products.
FORCEFIELD Source of rare earth magnets.
Laser Surplus Optical lab outfitter
Action Electronics Source of inexpensive CCD cameras. *Out of Business*
NightFire Electronic Kits Lots of SMT parts assembled as kits.
RapidLED Cree and Philips high power leds pre-soldered to stars.