Running Minecraft 1.8.1 on Debian Wheezy. Any quick and easy way to call an event, or is there a callback I can hook into for when users log in to the server?
Minecraft for PC doesn't provide such a native facility (though Minecraft for the Raspberry Pi does, with its integrated Python interpreter).
To do this, you can use pipe redirection with a log watcher or launch the server within a command wrapper that offers this kind of function. There are many options for this kind of functionality — none that are gaming specific — that you might already know about if you are familiar with *nix system administration, but if not Unix & Linux SE is likely the best source of further information.