http://www.deadmansswitch.net/ is what I wrote years ago to do it. Make sure your messages don't scare people too much, in case you forget to check in/don't get the emails, though.
You can customize your notification intervals to make them longer or more frequent, if you want. The risk isn't very big, though, especially if you remember to log in every first of the month, for example, just to make sure.