Is there a service available anywhere that sends out emails when various major programs are upgraded?
Good greif, do you have any idea what that would entail?
However: many distros have their own web sites and specialist forums which will showcase major upgrades important to that distribution. Similarily, developer sites will often tell you about latest releases.
For eg. When Mozilla releases new versions of Firefox and Thunderbird, this is announced on mozilla.org ... and is quickly picked up elsewhere. If nothing else, you'll know when Firefox 1.6 comes out from all the posts saying "I can't get fx 1.6 to connect to the internet".
Anyway - I understand Mozilla has a mailing list too.