If I understand correctly, the Buddha never wrote anything but some Buddhist texts quote him by describing meetings or situations where he was present. Is there a comprehensive list of all scripts where he is directly and textually quoted?
The Pāli Canon (Pali: Tipitaka) is the standard collection of scriptures in the Theravada Buddhist tradition, as preserved in the Pāli language. It is the most complete extant early Buddhist canon. It was composed in North India, and preserved orally until it was committed to writing during the Fourth Buddhist Council in Sri Lanka in 29 BCE, approximately four hundred and fifty four years after the death of Gautama Buddha.
See Pāli Canon on Wikipedia, also called the Tripiṭaka or Tipitaka, meaning "Three Baskets." I would recommend these books as a good reference to access original Buddhism. Here is an example from the Mulapariyaya Sutta, Majjima Nikaya section of the Pali Canon.
I have heard that on one occasion the Blessed One was staying at Ukkattha, in the shade of a royal Sal tree in the Very Blessed Grove. There he addressed the monks, "Monks!"
"Yes, lord," the monks responded.
The Blessed One said, "Monks, I will teach you the sequence of the root of all phenomena [or: the root sequence of all phenomena]. Listen & pay close attention. I will speak."
"As you say, sir," they responded.
This is an example of how each "Sutta" starts.
Within Buddhism there is the notion of Buddhavacana which is the word of the Buddha i.e. those world traditionally attributed to the Buddha. A lot of the Pali Canon does start with an attributation directly from the Buddha e.g. the Sigalovada Sutta sutra begins
This is what I heard.
But some other parts of the Pali canon doesn't read like that. The Dhammapada which is a good candidate for the original word of the Buddha in a historic sense doesn't read like that at all e.g. from the Twin Verses section at the beginning (brilliant incidentally)
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.
And other later words do start with a direct attribution to the Buddha but one would need to be very willing to accept the supernatural aspect of Buddhism if you were to believe that these represent actual historical events e.g. the White Lotus Sutra begins
This is what I heard. At one time the Buddha was in.....
So a lot of texts are attributed to the Buddha in a religious sense (the White Lotus sutra) but aren't actually good candidates for the historic word of the Buddha. Historically the Pali Canon is the best bet for that (like senshin says) but the word of the Buddha can be given a looser interpretation - or if you like a more devotional aspect.