The Aviation Safety Letter (ASL) is a Transport Canada publication that is mailed to every pilot, controller, and aircraft mechanic in Canada (and I’m sure elsewhere).
Starting with issue 1/2012 they will be publishing the letter online only. You will no longer get a paper copy in the mail.
I personally enjoyed getting the hard copy every quarter. It made good reading material on the train, bus, streetcar, etc…
If you would like to continue receiving the ASL (albeit the digital copy going forward) you will need to fill out the form at the Transport Canada website.
Sadly, the current issue is not on their website, yet I got it in the mail awhile ago.