I often find myself checking Hackage for updates to some packages. If there is a new release, I check the CHANGELOG or the homepage of the package for more information, and take action in my codebases if required.
Is there a tool or service where I can select some packages of interest and get notification emails when they are updated?
I understand that the new Hackage server does it, but only for packages that I am a maintainer of.
You can do that using RSS.
Go to the package of your choice in Hackage, click the RSS link and copy it in your feed reader.
The feed item will contain the number of the new release, who uploaded it, and the changelog in full.
There are many feed readers to choose from. I prefer ones running locally, but there are online readers too and many of those can send notification emails.
Also for the packages that are dependencies of the packages you maintain. So the most direct way to get those notifications would be to publish your own package.