[cherrypepsi]

On a PC, Arduino flashed with Firmata. It lets you write behavior in whatever language you want. It also keeps the serial connection open at all times so you can send commands as you go (e.g. with a Python REPL)

USB-to-serial still exists but the support for it is kind of crimped now due to security concerns (in Windows at least) and you can also damage the PC if the external circuit is not thought out correctly.

Not really sure if the USB adapter manages to isolate enough of the risk though, I have only used Firmata...

[Piskvorrr]

USB-to-serial has, first and foremost, horrible jitter issues.