Woodpecker is a utility that sends you a notification by texting your cellphone whenever someone logs into a Minecraft server. What’s special about this utility is that it pings the server, so it doesn’t have to be your own server and you don’t need any special admin permissions to see who’s on this server.
Even better, you can be banned or there can be a whitelist and this utility will still work!
To set up, check out the usage page.
Woodpecker is licensed under the MIT License. See details here.