On Windows (NT?) you used to be able to send a message across the LAN which would just pop up in a message box. Can't remember the command. Probably doesn't work anymore anyway.
Software recommendations
Do you want to know the best program to do something? Ask it here and discover the best options to choose between. Do not be shy
Anyone can ask for products by making a post. There are no posts because I think people think that they can not post but they can now
Want to get recommendations to non-software topics? Go to Recommendations
rules: instance rules
Netsend, I get the feeling it's blocked by default now
IIRC, it uses NetBEUI/NetBIOS, can't realistically block it. (You could, but it's not easy, and there's little value in doing so).
But it's not been included in windows since about Vista. But it still works. Just gotta get a version from Win2k.
Edit: I was thinking of Windows Chat.
Edit2: Net Send was replaced by MSG
Oh! Duh! I knew I was forgetting something. Yeah, Windows/Android.
I haven't tried it, but Kouchat looks interesting
Oh hey, going there reminded me at some point I'd found this and bookmarked it but haven't tried it yet, so thanks for the reminder! I should finally give it a try.
https://briarproject.org/ It can use bluetooth, wi-fi and internet to send text messages. Also has a desktop client.
Thanks!