Debugers have been already mentioned, but antiviruses do that, as well as various cheat and anti-cheat engines.

From a security perspective, reading/writing another program's memory space is well understood as a specific, risky privilege, so in all modern OSes control and prevent that if it's not "your" process or unless you're root/admin already.

But a cool thing for reading/writing another program's memory space is the ability to do that through PCI without the OS being able to intervene - e.g. Ulf Frisk's PCILeech (https://github.com/ufrisk/pcileech/), here's a video demo of writing to Notepad's memory remotely https://youtu.be/5DbQr3Zo-XY?t=1440