subscribe to nodes with entity reference I'm building a music site and have an 'artist' content type that are referenced in product displays, articles etc. I want to enable users to subscribe to these artists and get notified whenever there's a product display, article, event etc. that references the artist.

I've been looking into the Message Stack module to achieve this (http://www.jacksonriver.com/news/message-stack), but haven't been able to make it work with entity reference. 

Is this something anyone feels like helping me out with? Could be a very powerful feature IMO.
Try flags + Rules + Views

I use something similar on a jobs site to allow users to subscribe to 'industries' (taxonomy) and be notified when a new job tagged with the industry is created.

In short:
install the Flag module, and create a flag for taxonomy terms.
This flag, when clicked, would fire a rule saying "Subscribe the user to this term" and this would happen with a nice Ajax link that does not reload the page.

Create another rule, saying something like "When new content is posted in X term, email all users who have flagged (subscribed to) that term."

Then build a couple of views:
A page of all the terms (if you need this!), showing the Flag links (these can be text, or you can style them as nice images using CSS).
The second view: A block, emulating a Subscriptions block - basically this block would appear on your 'Artists' pages, and it would show the terms together with their flag/subscribe links.

You could build a third block, to be shown on the user's profile page, listing the terms they're subscribed to.
You might want to use the Context module to control the logic of where the blocks appear.