AVR microcontrollers come with some RAM already built inside chip. But what if you need more. If you select proper AVR like Atmega128, there is a XMEM interface that can be used to control external RAM. Simply speaking you can have up to 64Kbytes instead 4Kbytes.
Scienceprog has prepared simple tutorial how to add additional 8Kbytes of SRAM to Atmega128 board. He analyzes the ways this RAM can be uses and provides real test C routines where external RAM is tested. So, next time you run out of RAM think of adding more.