Delay: pausing a program for a fixed time with Delay.millis, Delay.micros and Delay.seconds.
Serial: printing and reading over USB serial, the main way to see what a program is doing on a board.
Gpio: driving and reading the board's pins, with DigitalOutput and the Blink example.
IMU: reading the UNO WiFi Rev2's onboard accelerometer and gyroscope with ArduinoUno.getImu(), over the Spi bus from ArduinoUno.getSpi(); see the UNO WiFi Rev2 page.
SD Card: loading configuration and credentials from a microSD card at boot, and reading and writing files.
Wifi: joining a Wi-Fi network on the boards that have a radio, or on a UNO R3 with the Wi-Fi shield.
Time: reading UTC with LocalDateTime.now() over SNTP, and measuring elapsed time with Clock.
HTTP client: HTTP and HTTPS requests (GET, POST, PATCH, DELETE and QUERY) with caller-owned buffers, and reading JSON responses.
HTTP server: serving HTTP and HTTPS requests from a program, with routing by path and a certificate on the SD card.
Email: sending mail over SMTP with Smtp, and reading a mailbox over POP3 with Pop3Client.
MVC: organizing a multi-screen app with a model, views, a navigator and a router from util.mvc.