ATtiny internal oscillator (OSCCAL) runtime calibration for UART Serial
This code originated as an attachment to the paper AVR054: Run-time calibration of the internal RC oscillator. It has been adapted to work with ATtiny84 and ATtiny85, along with example Arduino sketches.
The original source can be found by going to the tinyAVR Microcontrollers Documents page, choosing the "Application Notes" document type from the dropdown menu, and then searching for AVR054. The ZIP file image links to the code.