I was wondering about the name of the iconic Salzburg restaurant Skt Peters Stiftskeller. I looked it up on google translate and it says that means Saint Peters "pin celler". What's a pin celler? Why is this 1300 year old restaurant named that?

One possible derivation:

http://en.wikipedia.org/wiki/Stift

Stift is a monastery. Keller is a cellar. So the name globally means "St. Peters' monastery cellar".

(By the way, Stift may mean also a pencil, but this is not the meaning we are looking for.)

Over my time here in Vienna I have come to learn that the word "Stift" is derived from the German word "stiften," which means to donate. Many large monasteries across Europe were once endowed; hence, the term "Stift" is often associated with an ecclesiastical endowment. Think Stift Melk (Austria), for example; or, in this case, the,Sankt Peter Stiftskeller, the endowed monastery in Salzburg.

If you allow a bit of hair-splitting..
Stift does mean that it had been a donation (mostly by wealthy royalty), but it designates not only the house of worship, which can be a simple church or a large monastery - but, when applicable, also the whole estate which had been donated. Which often included acreage, farms, anxilliary buildings, breweries, vineyards etc. to support the church or monks/nuns and to make a little profit.
So it is more a word for "estate donated for religious purposes".
The phrase itself is more common in Austria than in Germany, though.

A Stift is, strictly speaking, not a monastery of monks or nuns, but a convent of worldly canons - there is a difference in the vows they have to take, they do not take the full profession and they are not part of any monastic order. In Austria, however, the term is also in use for monasteries (in Germany it is not). St Peter in Salzburg is indeed a monastery of the Benedictine order.

I assume that you live in a protestant part of Germany
Now that you mentioned it, I do remember that in the protestant parts of Germany (as well as neighboring Netherlands, for example), a Stift is more known as a worldly canon like you described it - often called a Damenstift.
In the Southern parts of Germany and even more Austria, a Stift is hardly ever such a community,

It's a cellar below a major church - which was apparently once a monastery. The whole look is sort of cave-like and it's very atmospheric.

When we were there the food was very hearty and traditional and they had a musical group - lead by a zither.

So where do I sign up to be a worldly canon? Not sure what exactly it entails but it's got to beat the stuffing out of poverty, chastity and obedience.

Seamus, i'll be right behind you in line to be a worldly canon, especially if it means we get to eat at Skt Peters Stiftskeller frequently. It is a "must do" whenever we get to Salzburg.

Canons were "wordly" in the sense that they were not monks, but priests mostly working in the cathedral and entrusted with the task electing the archbishop (that till early 19th century was also the ruler of Salzburg). At the present time the archbishop is appointed by the Vatican, but in respect of the canons tradition, the Pope will stick to one of three names suggested by the canons group.

Wikipedia knew it all the time
http://en.m.wikipedia.org/wiki/Stift

You find the possible meanings ranked by usage or historic/regional development of the meaning.
So if you asked someone from Northern Germany or Holland what a Stift or sticht was, he would think of the worldly canon for typically unmarried Protestant women.
And if you asked someone from Austria or the Catholic South where there had been no Reformation, he would think of the aforementioned endowed monasteries or other ecclestic estates first.

The Stiftskeller in Salzburg is probably a good location to meditate about how historic events a few centuries ago can still have an impact on modern day use of language - preferrably after a few glass of the local brew

Cowboy you are again mixing up things. Ever heard of a Domstift, Stiftsherren, Kanonikerstift etcetera? A "Stift" (in the German use of the word) has a different status than a monastery and canons are not monks. In Southern Germany a monastery is not a "Stift".

A protestant Damenstift is an entirely different matter.

I live, by the way, in Baden-Württemberg.

In Switzerland, a "Stift" is an apprentice. It's also a foundation for a worthy cause. The foundation can be religious-based or not.

I didn't mean to start the holy wars. I assume is is just a restaurant now.