The big one: Thomas' commentary on Book VII of Aristotle's Metaphysics. As with all the commentaries here, it is closely linked to Aristotle's text. In this case, William of Moerbeck's Latin translation from the Greek, in parallel with Ross's English translation from the Greek. The text also includes links to Averroes' commentary on the Metaphysics, in the Latin that translated from the Arabic (from an edition published in Venice in 1562), also links to a 14th century manuscript of William's translation.
Book VII is at the heart of the Metaphysics. It is very difficult to understand. Thomas's commentary is usually very clear, and helps a bit.