Of God and His Creatures  
   An annotated, abridged version of St. Thomas Aquinas's work "Summa Contra Gentiles". At Notre Dame.
   http://www.nd.edu/Departments/Maritain/etext/gc.htm

On the Mixture of the Elements, to Master Philip of Castrocaeli  
   Paul Vincent Spade translation. In pdf. 111K.
   http://pvspade.com/Logic/docs/mixture.pdf

Summa Theologica  
   Complete text of the 1947 Benziger Brothers edition, with concordances and word frequency lists. Each question in its own file.
   http://www.intratext.com/X/ENG0023.htm

On the Eternity of the World  
   Robert T. Miller translation. "We thus ought to determine whether there is any contradiction between these two ideas, namely, to be made by God and to have always existed." At the Internet Medieval Sourcebook.
   http://www.fordham.edu/halsall/basis/aquinas-eternity.html

Aquinas on Law  
   Selections from the Summa Theologiae pertaining to law, translated by David Burr. "Responsio" sections only, some articles omitted entirely. From I-II, 90, 1, to I-II, 97, 2.
   http://www.fordham.edu/halsall/source/aquinas2.html

On the Principles of Nature  
   Gerry Campbell translation.
   http://www4.desales.edu/~philtheo/aquinas/Nature.html