There is an RSS module that's part of the EE base install. You may have to turn it on, but it's there. You can set up RSS feeds for any category or channel you wish, and it can contain a URL, the whole content...whatever. Here's the user manual link to using the RSS module (note that it is for EE version 2.7+, but the directions should be fairly similar even for older versions. I don't know that the RSS module has had any feature additions since version 1.5x, anyway.)

People would not have to be EE users to get the RSS feeds. All they'd need is an RSS reader.

There's also a mailing list module with the default EE install, for email lists - but I think that's for building an email list and communicating with them rather than for automatically notifying when there are changes.

I know that there are other modules out there, created by third-party developers and companies. devot-ee lists them and includes ratings on the modules themselves.