Is there any good summary of comma and interpunctuation rules? I know that English spelling traditionally requires fewer commas than, for example, German, however I am often unsure whether to use a comma or not.
In English (as opposed to German, where the comma rules are based on syntax), commas are usually deployed to indicate intonation dips.
This is why commas are required with non-restrictive relative clauses, for instance, because the two relative clause types are distinguished in speech by intonation differences.
So the rule in English is to use a comma whenever you intend the reader to "hear" a Mid-High-Low-Mid intonation sequence, as if you were speaking it aloud. This is phonological, not grammatical, rather like the English rules for using articles before consonants and vowels.
There are more grammatical approaches to comma usage in English, though as John Lawler indicates, choice can depend on when one wishes the reader to 'hear' that particular intonation sequence. It can also depend on when you would like the reader to pause in reading a sentence.
More grammatical reasons for the use of a comma can be found at:
http://www.grammar-monster.com/lessons/commas_in_lists.htm (and related grammar-monster pages).