Online UART bitrate calculator for AVR-microcontrollers.
By default, you can get a list of the UBRR settings for many popular clock speeds, and serial port bit rates.
But you can tell it what serial port bit rate you want, and it will tell you all the possible clock speeds that will
generate your desired serial port bit rate.
You can also give it a clock speed, and it will tell you all the possible serial port bit rates that can be producted with that clock speed.