In the centuries immediately following the Buddha's death and the First Buddhist Council, arahants and scholar-monks began recording their own commentaries to the teachings of the Tipitaka. At first these commentaries, explanations, amplifications, meditation notes, and historical recollections were passed down orally within the monastic sangha but, like the Tipitaka itself, many began to be recorded in writing around the turn of the Common Era. Most of these early texts — primarily written in Sinhala — remained for centuries tucked away in forest monasteries and temples in Sri Lanka, accessible only to a few Sinhala scholars. It wasn't until these scattered fragments were translated into Pali and collated into coherent texts (most notably by the great Indian scholar Buddhaghosa (5th c)) that they became available to the wider Theravadan world.
Since then, these texts — variously labeled "non-canonical," "extra-canonical, or "post-canonical" — have come to be regarded as essential supplements to the teachings of the Pali canon itself. So treasured, for example, is the Milindapañha that it has even been subsumed in the Burmese edition of the Tipitaka; and in some parts of the Theravadan world Buddhaghosa's monumental Visuddhimagga is regarded as a more definitive guide to Buddhist meditation practice than even the Tipitaka itself. Taken together, the Tipitaka and much of this non-canonical Pali literature (in particular the Commentaries) constitute the complete body of classical Theravada texts.
For a more detailed overview, see "Beyond the Tipitaka: A Field Guide to Post-canonical Pali Literature."
At present Access to Insight offers translations of only a handful of non-canonical Pali texts. I hope to add more in the years to come.