Adafruit. Full range of devices, sensors, and software (including CircuitPython).
DFRobot. Producer of open source hardware and software products, including a full range of sensors, with a strong focus on developers and STEM education.
National Control Devices. A producer of IoT devices, software, and services for industrial and military applications. Includes feather boards for 5 volt expansion of I2C communications.
Particle.io. A producer of IoT devices and services with a larger developer community.
SeeedStudio. A producer of a IoT devices, sensors, and development kits. Produces a wide range of "Grove" sensors which use standardized connector and modules to simplify development of IoT applications.