It would be nice if Linode would offer a service to monitor selected services (SSH, HTTP, HTTPS, IMAP, SMTP, etc.) and provide services to page/email me when a service is down.

There's a bajillion services like that out there already, pingdom.com, cloudkick.com etc etc, you can also install something like monit.

Yeah in this case too there's a big advantage in using an outside service since it's more likely to function if there's any kind of linode infrastructure outage that kills both your node and whatever linode would use for monitoring.