MightyCore is designed to bring Arduino capabilities to a number of ATmel processors not originally supported by the platform. It was developed by MCUdude and all rights are held by the developer.
Supported Processors include:
And that last one exists in PLCC44.
MightyCore on GitHub: https://github.com/MCUdude/MightyCore
There are various reasons you would want to go with a different processor - more timers, more PWM outputs, or more straight I/O Pins to do some kind of complex display of data for which your standard 13 just aren't enough.