The website http://ifttt.com, which stands for "If This Then That," allows you to makes things happen online as a result of something else happening. The If/Then is a reference to logical causality, and in this case basically means,"if this one thing happens on the internet, then do this other thing automatically."
They explain it very well on their "About" page (I put "About" in quotes because their actual URL made me laugh and is so much better than "/about").
Anyway, there already are some tools that offer consequence-action services (like Google Alerts, getting an email if someone comments on your flickr photos, using Twitterfeed to automatically tweet blog posts, etc). But this one seems the most versatile, because it isn't service-dependent, it does more than just notifications, and it lets you manage all your notifications from one service.
I'm hoping to use it to automate some of what the library does online (as seen in our Online Marketing Flowchart). There are lots of triggers and actions available, and it seems limited only by your imagination. But of course, like with any online tool, the more you use it, the bigger impact you'll feel if it suddenly goes away - which never stopped me before.
Also, like LibraryElf, this is a tool I think patrons can use on an individual basis - I say this because it offers notifications by text, phone, and email, and triggers can be calendar events, feeds, and more.