The Etymologies (Latin: Etymologiae) or Origines is an encyclopedia compiled by St Isidore of Seville in the early seventh century. It was a major source for the transmission of classical learning to the Middle Ages, partly because of its relatively simple Latin. Modern students of Latin may find it worth dipping into for the same reason.
Free online texts
Internet Archive: Isidori Hispalensis episcopi Etymologiarum sive Originvm libri XX, Volume 1. Latin text, multiple formats.
Intratext: Etymologiarum sive originum libri XX, edited by W. M. Lindsay (Oxford, 1911). Latin text, HTML format.
LacusCurtius: The Etymologies (or Origins) - Latin text, html format.
The Latin Library: Etymologiarum sive Originum Libri XX - Latin text, html format.
Wikisource: Etymologiarum libri XX - Latin text.
Bestiary.ca: An Encyclopedist of the Dark Ages - Isidore of Seville, by Ernest Brehaut, Studies in History, Economics and Public Law, Columbia University, 1912. PDF format.
British Library: Medieval Manuscripts Blog - Isidore of Seville's Etymologies: Who's Your Daddy?
The Great Conversation: Further reading at Tom's Learning Notes
Pliny: Natural History.
Latin Resources: Online materials for learning Latin.