What is ACARS? ACARS is a VHF digital system used to communicate between the aircraft and the ground. It can be thought of as "e-mail for airplanes".
I'll get more info as I find out more... this is new to me and I want to make sure the information I put on the page is correct.
Here's the frequencies that are used to transmit ACARS data to and from Aircraft.(see update below)
131.550 Primary Channel Worldwide
130.025 Secondary Channel for US and Canada
130.425 US Channel
130.450 US + Canada
131.450 Primary for Japan
131.475 Air Canada Company Channel
131.725 European Primary
131.525 European Secondary
136.925 ARINC European Channel
136.850 SITA North American Frequency
UPDATE: The following frequencies were published in the April Popular Communications Magazine as the main frequencies for US and Canada.