How do I see all iOS in-app subscriptions that I have active? If I subscribe to several iOS in-app subscriptions, such as magazine app subscriptions on my iPad, then how can I later show a list of all my in-app subscriptions?
This article by Federico Viticci on Mac Stories gives a detailed description of how iTunes subscriptions work. In particular, he provides a link to a page to manage subscriptions which you can open on an iPad (and presumably an iPhone or iPod touch):

As soon as I subscribed to The Daily, Apple sent me an email to confirm the Weekly Subscription.
I clicked on the link provided in the email, which looks something like this.

This page allows you to see all of your existing subscriptions, as well as to manage or unsubscribe from them.
It’s worth noting, however, that the article in question was written almost two years ago (February 2011). He states:

The browser tried to launch iTunes on my Mac, but iTunes returned the following error:
Subscription management: Please use your iPad to Manage your Subscriptions
Apparently, Apple’s current implementation of subscriptions can be managed only on the iPad.

Since then, this issue has been closed. Clicking the link on a Mac now takes you to a page to manage your subscription in iTunes, in addition to using an iOS device.