Some of the writings of many of history's most important philosophers are available online for free download. Please see the following partial list of such books:
Plato's Writings:
Volume 1 ; Volume 2 ; Volume 3 ; Volume 4 ; Volume 5 ; Volume 6.
Boethius:
The Consolation of Philosophy
The Writings of Philo of Alexandria (Philo Judaeus):
Volume 1 ; Volume 2 ; Volume 3 ; Volume 4.
Saint Augustine:
Volume 1 ; Volume 2 ; Volume 3 ; Volume 4 ; Volume 5 ; Volume 6 ; Volume 7 ; Volume 8.
Immanuel Kant:
The Writings of David Hume:
Volume 1 ; Volume 2 ; Volume 3 ; Volume 4.
The Writings of John Locke:
Volume 1 ; Volume 2 ; Volume 3 ; Volume 4 ; Volume 5 ; Volume 6 ; Volume 7 ; Volume 8 ; Volume 9 (not available) ; Volume 10.
Confucius
The Sayings of Confucius