Gildas' De Excidio et Conquestu Britanniae (On the Ruin and Conquest of Britain) is a unique document of post-Roman Britain. Probably composed some time in the sixth century, it is not so much a work of history, as a polemic against various sub-Roman kings, whose impiety and misrule is blamed for the misfortunes of the Britons and the conquests of the Anglo-Saxons. The paucity of documents from this disturbed period of British history mean it is nevertheless an invaluable source.
Free online texts
Heroofcamelot.com: Gildas' On the Ruin of Britain, translated by J.A. Giles and T. Habington. Creative Commons PDF file.
Gutenberg: On the Ruin of Britain, translated by J.A. Giles. Multiple formats.
Internet Archive: Gildas: The Ruin of Britain, Fragments from Lost Letters, the Penitential, together with the Lorica of Gildas. Edited for the Hon. Society of Cymmrodorion by Hugh Williams (London, 1899). Latin and English, multiple formats.
Internet Archive: The works of Gildas and Nennius, translated by J.A. Giles (1841). Multiple formats.
KMatthews.org.uk: Gildas de excidio et conquestu britanniae. Latin text, HTML files.
Tertullian.org: The Ruin of Britain, English text edited (translated?) by Hugh Williams (1899).
Wikisource: The Ruin of Britain (6th century), by Gildas, translated by Thomas Habington and John Allen Giles. HTML file with PDF/EPUB/MOBI downloads.
Internet Archive: Of the Ruin of Britain (De excidio Britanniae) - public domain audiobook.
Internet History Sourcebook: The Life of Gildas, by Caradoc of Llangfarn.
The Great Conversation: Further reading at Tom's Learning Notes
Bede: Ecclesiastical History of the English People - relies heavily on Gildas as a source on the Anglo-Saxon invasions.
Nennius: History of the Britons.