Hermetic Philosophy and the Mystery of Being
A fascinating compendium of information about the timeless Perennial Philosophy of Plotinus and so many other sages of East and West.

Plotinus
The Stanford Encyclopedia of Philosophy has a clear synopsis of Plotinus's teachings written by Lloyd Gerson, a noted Plotinus scholar.

Plotinus
An easy-to-read overview of Plotinus's metaphysics. This site also takes a look at Neoplatonism in general, plus an impressive array of other esoteric topics.

Enneads
This translation of Plotinus's writings, the "Enneads," is by Stephen MacKenna and has been superceded by A.H. Armstrong's more recent translation. But MacKenna's version isn't copyrighted, so can be viewed in its entirety for free.