Periodically updated
Today is BlackFriday, sellers offer discounts for a lot of products. Here is list of them.

RAK Wireless
10% off for RAK831 Pilot Gateway (RaspberryPi 3, GPS, modul s RAK831). From 199 to 179.10 USD.

https://www.aliexpress.com/store/product/RAK831-Pilot-Gateway-SX1301-Raspberry-Pi3-GPS-with-complete-enclosure-for-professional-demonstration-set-up/2805180_32883987457.html

IQRF.shop (Ivona Spurná, FB)

Discount 50% for multipacks DS-START-04, DS-DPA-02 and DS-IoT-01. Use IQRFNOV50 code.

https://iqrf.shop/

Adafruit.com

Discount 20% for products from FEATHER family. For other products adafruit offers 15% off.

https://blog.adafruit.com/2018/11/23/this-black-friday-its-feather-friday-at-adafruit-use-the-code-blackfruit-to-get-20-off-feathers-and-15-off-items-storewide/

Pimoroni.com
Camera for RaspberryPi (Zero) (25%), pHAT DAC (38%), micro:bit complete starter kit (20%), Inky pHAT (ePaper, Eink,EPD) (25%), Sensor for micro:bit (25%), Thermal Camera breakout (20%) and more.

https://shop.pimoroni.com/collections/special-offers/

Hardkernel.com
Odroid -XU4 and Odroid-XU4Q for 49 USD.

XU4: https://www.hardkernel.com/shop/odroid-xu4-special-price-limited-quantity/
XU4Q: https://www.hardkernel.com/shop/odroid-xu4q-special-price-limited-quantity/

ThePiHut.com
Discount from 5% to 50%.
RaspberryPi 3B (5%), Camera for RaspberryPi (5%), RaspberryPi 3B Starter Kit (10%), ZeroCam for RPi Zero (10%), Enviro pHAT (20%), Adafruit PiTFT Plus LCD (20%) and more.

https://thepihut.com/collections/raspberry-pi-sale-items

Tindie.com (Radek Zima, FB)
LoRa Tracker (79.95 to 59.95 USD), TinyFPGA BX (38 to 28 USD), DSTIKE WiFi Packet MonitorV3 (19 to 16.15 USD) ESP-RFID Realy Board (15 to 13.50 USD), EVSE Tester (40 to 34 USD) and more.

https://www.tindie.com/browse/sale/

DFRobot.com
Gravity LCD shield (19.50 to 12 USD) , LCD for LattePanda (34 to 23 USD), 10.1" LCD with HDMI/AV/VGA (119 na 89 USD), 7" touch LCD (69 to 49 USD)and more.
You can also use these coupons
5% discount for purchase 1 USD and more - BF1805
10% discount for purchase 100 USD and more - BF1810
15% discount for purchase 200 USD and more - BF1815
20% discount for purchase 400 USD and more - BF1820

https://www.dfrobot.com/index.php?route=product/special

Arduino Store
Arduino Leonardo (20 to 15 Euro), Arduino Uno Rev3 (20 to 15 Euro), Arduino Micro (16 to 12 Euro), Arduino Starter-Kit (79.90 to 59.90 Euro), Arduino Due (35 to 26.50 USD), Arduino Mega 2560 Rev3 (35 to 26.50 Euro) and more.

https://store.arduino.cc/arduino-genuino/project-promotion

Pycom

Discount 20% with this coupon BLACKF2018. The coupon is not valid for multipacks.

https://pycom.io/webshop/

Seeedstudio
Arduino Uno Rev3 (24.95 to 19.96 USD), Grove - Starter Kit (49.90 to 39.92 USD), AlphaBot2 (59.90 to 50.99 USD), Cactus Micro Rev2 Arduino compatible plus esp8266 (11 to 2.99 USD), 2.2" LCD for RaspberryPi (22.95 to 13.77 USD), WiFi XBee (48.75 to 29.25 USD), LoPy (LoRa + Pycom) (28.90 to 19.20 USD), Espruino Pico (29.99 to 27.49 USD), RaspberryPi Compute Module (35 to 21 USD), DSO Quad pocket oscilloscope (199 to 119 USD) and more.

https://www.seeedstudio.com/act/flashsale-17.html

Sparkfun.com
Discount 20% for everthing.

https://www.sparkfun.com/news/2827