TCP/UDP Applicaton for UNO/MEGA/STM32 using ESP8266's AT firmware.
You don't need Ethernet card.
You need only ESP8266 module.

You can save flash & SRAM

UNO+ENC28J60+UIPEthernet+PubSubClient

Sketch uses 25090 bytes (77%) of program storage space. Maximum is 32256 bytes.
Global variables use 1736 bytes (84%) of dynamic memory, leaving 312 bytes for local variables. Maximum is 2048 bytes.
Low memory available, stability problems may occur.

UNO+W5100+Ethernet+PubSubClient

Sketch uses 19582 bytes (60%) of program storage space. Maximum is 32256 bytes.
Global variables use 1101 bytes (53%) of dynamic memory, leaving 947 bytes for local variables. Maximum is 2048 bytes.

UNO+ESP01+Software Serial

Sketch uses 9938 bytes (30%) of program storage space. Maximum is 32256 bytes.
Global variables use 897 bytes (43%) of dynamic memory, leaving 1151 bytes for local variables. Maximum is 2048 bytes.

MQTT_Publish_ESP01
Simple Pubish Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.

MQTT_Subscribe_ESP01
Simple Subscribe Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.

Socket_Client_ESP01
Simple TCP/IP Client Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.

Socket_Server_ESP01
Simple TCP/IP Server Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.

Socket_Client_WiFiEsp
Simple TCP/IP Client Application using WiFiEsp.
Supprted UNO/MEGA.
It require Arduino WiFi library for ESP8266 modules.
https://github.com/bportaluri/WiFiEsp

Socket_Server_WiFiEsp
Simple TCP/IP Server Application using WiFiEsp.
Supprted UNO/MEGA.
It require Arduino WiFi library for ESP8266 modules.
https://github.com/bportaluri/WiFiEsp

NTP_Client_ESP01
Simple NTP Client Application.
Supprted UNO/MEGA/STM32.
It require Time library for Arduino.
https://github.com/PaulStoffregen/Time

SNTP_Client_ESP01
Simple SNTP Client Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.

SMTP_Client_gmail_ESP01
Simple SMTP Client Application.
Supprted UNO/MEGA/STM32.
It DON'T require any library.
You need gmail account.

I'm using esp8266_flasher.exe and v2.0 AT Firmware(ESP).bin.

Connect to ESP-01 with 115200 bps.

AT+GMR
AT version:0.40.0.0(Aug  8 2015 14:45:58)
SDK version:1.3.0
Ai-Thinker Technology Co.,Ltd.
Build:1.3.0.2 Sep 11 2015 11:48:04
OK

AT+CWMODE=1

OK
AT+CWLAP
+CWLAP:(3,"Picking",-86,"34:12:98:08:4b:4a",1,-4)
+CWLAP:(4,"ctc-g-fa4a2e",-92,"c0:25:a2:b1:8c:2e",2,3)
+CWLAP:(4,"aterm-e625c0-g",-49,"c0:25:a2:ac:cb:ba",3,15)
+CWLAP:(1,"aterm-e625c0-gw",-48,"c2:25:a2:ac:cb:ba",3,15)

OK

AT+CWJAP="Your AP's SSID","Your AP's password"
WIFI CONNECTED
WIFI GOT IP

OK

AT+CIPSTA?
+CIPSTA:ip:"192.168.10.142"
+CIPSTA:gateway:"192.168.10.1"
+CIPSTA:netmask:"255.255.255.0"

OK
AT+CWQAP

OK

WIFI DISCONNECT

*** UNO ONLY ***
*** Change baudrate to 4800bps ***
*** Because there is no the 2nd UART in UNO ***
*** So UNO use Software Serial with low speed ***

AT+UART_DEF=4800,8,1,0,0

Re-Connect to ESP-01 with 4800 bps.

ESP-01(Tx) - UNO(D4)
ESP-01(Rx) - UNO(D5)

You can't use on-board 3.3V.
An electric current is insufficient.

ESP-01(Tx) - MEGA(D19)
ESP-01(Rx) - MEGA(D18)

You can't use on-board 3.3V.
An electric current is insufficient.

ESP-01(Tx) - STM32F103(PA3)
ESP-01(Rx) - STM32F103(PA2)

MAPLE Core.
https://github.com/rogerclarkmelbourne/Arduino_STM32

ESP-01(Tx) - STM32F103(PA10)
ESP-01(Rx) - STM32F103(PA9)

ST Core.
https://github.com/stm32duino/Arduino_Core_STM32

ESP-01(Tx) - STM32F103(PA3)
ESP-01(Rx) - STM32F103(PA2)

Serial printing goes to PA9.

ESP-01(Tx) - STM32F103(PA3)
ESP-01(Rx) - STM32F103(PA2)

Serial printing goes to PA9.

ESP-01(Tx) - STM32F103(PA3)
ESP-01(Rx) - STM32F103(PA2)

Serial printing goes to PA9.

ESP-01(Tx) - STM32F303(PA3)
ESP-01(Rx) - STM32F303(PA2)

Serial printing goes to PA9.

ESP-01(Tx) - STM32F401(PA3)
ESP-01(Rx) - STM32F401(PA2)

Serial printing goes to PA9.

ESP-01(Tx) - STM32F4DISC1(PD9)
ESP-01(Rx) - STM32F4DISC1(PD8)

I want to Fritzing Part of this board.
Serial printing goes to PA2.

ESP-01(Tx) - STM32F407(PA3)
ESP-01(Rx) - STM32F407(PA2)

https://stm32-base.org/boards/STM32F407VGT6-diymore
Serial printing goes to PA9.

1.Make sure TE(terminal equipment) is in sta or sta+ap mode

AT+CWMODE=3
OK

2.Make sure TE got ip address

AT+CWJAP="ssid","12345678"
OK

AT+CIFSR
192.168.1.134

3.Let's update

AT+CIUPDATE
+CIPUPDATE:1    found server
+CIPUPDATE:2    connect server
+CIPUPDATE:3    got edition
+CIPUPDATE:4    start start

OK

We can get a module for UART communication.
We can write AT firmware to this.

Only $1 for aliexpress/eBay.
It has a 5V->3.3V regulator and can supply power directly from UNO's 5V.

UNO compatibles can have a pin header inside the pin socket.
If you do not use SD-CARD, you can freely use digital pins # 10 to # 13 of the TFT shield.
These pins can be used for UART communication with the ESP-01.