I'm interested in a site which still is in beta and has a very low traffic. I'd want to get notified as soon as a new question is posted on that site, since they are few. I know we can subscirbe to questions and tags, but I'd want to get an instant email when a new question is published on that specific SE site, with any tag. Is that possible?
Right now there is no way to get an email for each and every new question on a site. There are two other options you have though:
Use the rss feed. At the bottom of every site's homepage is a "recent questions feed" link which you can use in your favorite rss reader. Depending on the reader you use, you could get a notification similar to email.
Subscribe to the newsletter. However, this only gives a periodic email about the most popular posts from a given timeframe. This is not ideal for instant notification.