The atmega1284p is described by Maniacbug who made a perfect instruction of how to fit the atmega1284p with the arduino IDE.
There is now an optiboot version of the bootloader which gives you even more memory https://github.com/maniacbug/mighty-1284p/tree/master/bootloaders/optiboot
Also you can find the required changes to the arduino IDE called mighty1284 from the maniacbug site.
For me it all worked pretty much straight out of the box.
I have used it on an Sanguinololu board for making my pick and place unit.
This is a pretty good board allowing control of 4 motors and plenty of other IO which allows you to get all the sensors required by the pick and place unit.
You can find the pick and place unit here. I use the repetier firmware which i do find excellent. You can download it from https://github.com/repetier/Repetier-Firmware
This is a whole operating system for extrusion machines which i someday hope to adapt for pick and place.
If you are looking for other arduino projects look here.
Here is the pinbout of the Atmega1284p