The Loeb Classical Library series (LCL) comprises originals and translations of ancient Greek and Latin texts. Volumes feature the original Greek or Latin text on the left facing an English translation on the right.
A collection of the out-of-copyright Loeb volumes has been scanned and provided online by Bruce Robertson, Dept. of Classics, Mount Allison University and Federico Boschetti, ILC-CNR, Pisa.
Also, Edwin Donnelly’s DownLOEBables provides links to out-of-copyright Loeb volumes available on the internet.
h/t Chris Zeichmann