by Pliny The Elder
In my opinion, the Perseus Digital Library offers one of the best ways to read and search through Pliny's Natural History. It offers both the latin version—edited by Karl Friedrich Theodor Mayhoff (1841-1914)—as well as the English translation created by John Bostock (1773-1846).
The site makes it easy to navigate through each chapter and book, and offers an easy to search for keywords, either in Pliny's original text or the notes that were added later on. Aside from that, the latin version offers something extra, by clicking the words it offers up a translation, how often the word was used in the text, as well as a linguistic breakdown.