Using USB to control external device

Hello,
This is my first post.

I have been at ABB working with power transformers for the past 7 years, and before that I was in university studying operations management and working... Last time I was involved with "low level" electronics was like 15 years ago...USB wasn't as commonplace then. But I changed jobs about a year ago. I have been tasked with evaluating the feasibility of a new potential product. I need to make a "proof of concept" prototype. Here is my issue:

I need to control 3 channels, perhaps add a few spare ones for future needs, but we are talking of no more than 8. I want to control them with a computer trough the USB, using a simple program where you can simply switch any contact on or off. I would imagine that there are a lot of people using USB to control all kinds of devices, I am sure that they exist. But I don't know the correct terminology. Where can I find this and what is it called?

Since this is a prototype, I don't mind buying some form of development kit. Ideally a small simple card with insulated contacts that can switch 24 V and is powered itself from the USB.

USB itself is quite complicated, by FAR the easiest option is to use a USB to Serial converter, then use a PIC (or other micro-controller) to respond to the serial commands from the computer - the computer just needs to be running a standard serial comms program, or you can write your own specific program on the computer.

Probably the easiest, and cheapest, way is to evaluate it with an Arduino Uno - it's a development board for an AVR processor, and already includes the USB/Serial converter, and the Arduino IDE even includes a serial comms program so you can send commands to the Arduino, and receive responses back.

You can also buy 'shields' which plug directly in the top of the Uno, which are freely and cheaply available - including ones with multiple relays, which sounds perfect for what you want.

You can get ready-made USB relay / USB input-output units quite cheaply.

Just search "USB relay i/o" on ebay for a range.

These seem quite common:

Or from many hobby / robotics / home automation suppliers, eg.

There are also many Ethernet-connected types, which can be more versatile than USB - eg.

I can stop playing that U2 song on repeat because now I have found what I was looking for. Thanks a lot guys!