I don't know how I hadn't encountered it before, but I just noticed that the complete works of Leonhard Euler are online at Dartmouth. Here, for instance, is his famous paper on the bridges of Königsberg.

They're all reproduced as scans of the original Latin, but they also include pointers to translations and related papers by others, when available. In the case of the Königsberg paper, their page on it lists two English translations published in books (both of which I have), but there's also an online English translation here, with a slightly different selection of figures than the original from the archive but otherwise reasonably complete looking.