Thanks to the Library of Congress, many of Aesop’s fables are online in a gorgeous, interactive edition with illustrations by Milo Winter.
Google Books also has many electronic editions of Aesop collections, including a free public domain volume illustrated by Harrison Weir.
There’s a searchable collection at www.aesopfables.com, which also includes fairy tales from Hans Christian Andersen and others. This is likely the most extensive collection, and you’ll find many fables that are nearly identical but with slight variations in the details.
Who was Aesop anyway? Wikipedia can tell you a little more about who he might have been. But no writings attributed directly to “Aesop” survive. We do know that many philosophers and poets, including Aristophanes and Sophocles, knew of Aesop’s stories; Sophocles composed some of them into verse poems. We also know Aesop didn’t write down any of the morals; the lesson of each story was thought to be clear without articulating it, but later authors have added them and today we recognize many familiar aphorisms in the morals of “Aesop’s Fables.”
And for more images, head to Wikimedia Commons for a wealth of public domain illustrations from historical editions of the fables.