Opuscula can be found in vol. 40 - 43 of the Leonine edition of Opera Omnia.  Other editions are also available including the following selected works:

Opuscula Theologica
T.  Marietti, 1954 (Stacks, BQ 6831 .L35 1954)   

Opuscula Philosophica
Cura et studio Raymundi M. Spiazzi, 1954 (WRLC, B 765 T5 1954). 

De Principiis Naturae (1253)
Standard title: De principiis naturae ad fratrem Silvestrum.  Introduction and critical text by John J. Pauson.  Contains text in Latin and introduction in English.  (Stacks, BQ 6857 .P64 A2 1950).

De Regno, ad regem Cypri (1265-1266) 
Politica opuscula duo ad fidem optimarum editionum diligenter recusa Joseph Mathis curante. Edition: 2. ed. revisa (Stacks, BQ 6857 .R34 1948).

De Unitate Intellectus (1270)
Contains Latin text and English translation of Aquinas’s De unitate intellectus contra Averroistas.  (Stacks, BQ 6857 .U7 M34 1993)

De Ente et Essentia (1253)
Edidit Ludovicus Baur.  Includes bibliographical references (Stacks, BQ 25 .O62 S3 fasc.1)

De Aeternitate Mundi Contra Murmurantes
Translated works available in the library: An Aquinas Reader, pp. 178 - 185 (Stacks, BQ 6832 .R28).  Translated by Mary T. Clark.   A First Glance at St. Thomas Aquinas: a handbook for peeping Thomists, pp. 49 - 56.  Translated by Ralph McInerny.  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 49-89.

De Ente et Essentia  
Aquinas On Being and Essence. Translated by Joseph Bobik (Stacks, BQ 6857 .E57 E5 1965); Concerning Being and Essence.  Translated by George G. Leckie (Stacks, BQ 6857 .E57 E5 1937);  On being and Essence.  Translated by Timothy McDermott.  Available in Thomas Aquinas: selected philosophical writings, pp. 90 -113 (Stacks, BQ 6831.2 .E5 1993); On Being and Essence.  Translated by Ralph McInerny in Thomas Aquinas: Selected Writings, pp. 30 - 49.  Translated by Ralph McInerny (Stacks, BQ 6831 .E5 1998); On being and Essence. Translated with an introd. and notes by Armand Maurer (Stacks, BQ 6857 .E57 E5 1968).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 315-381.

De Principiis Naturae ad Fratrem Sylvestrum  
Aquinas on Matter and Form and the Elements: a translation and interpretation of the De principiis naturae and the De mixtione elementorum of St. Thomas Aquinas.  Translated by Joseph Bobik (Stacks, B765.T53 D744 1998);  Selected writings: The principles of nature, On being and essence, On the virtues in general, On free choice.  Translated by Robert Goodwin (Stacks, BQ 6831 .E5 1965).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 1-47. 

De Substantiis Separatis  
Other titles: De substantiis separatis ad fratrem Reginaldum socium suum; De Substantiis Separatis seu de Angelorum Natura.  Treatise on Separate Substances.  Based on tvelve mediaeval manuscripts by Francis J. Lescoe (Stacks, BQ 6857 .S86 E5 1959).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 40 D.  

De Unitate Intellectus Contra Averroistas
Translated works available in the library: The Trinity and The Unicity of the Intellect (Stacks, BQ 6860 .L44 E5 1946).  Translated by Sister Rose Emmanuella Brennan.  Aquinas Against the Averroists: on there being only one intellect (Stacks, BQ 6857 . U7 M34 1993).  Translated by Ralph McInerny.  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 243-314.

De Articulis Fideie 
Full title: De articulis Fidei et Ecclesiae Sacramentis ad Archiepiscopum Panormitanum.  The Catechetical Instructions of St. Thomas Aquinas, pp. 119 - 131.  Translated with a commentary by Rev. Joseph B. Collins with introduction by Rev. Rudolph G. Bandas (BQ 6831 .E5 1939).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 42, pp. 207-257.

Compendium Theologiae
Full title: Compendium Theologiae seu Brevis Compilatio Theologiae ad Fratrem Raynaldum.  Compendium of Theology.  Translated by Cyril Vollert (Stacks, BQ 6856 .C66 E5 1947).  Also Compendium of Theology.  Translated by Richard J. Regan (Stacks, BXZ 1749 .T36 2009).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 42, pp. 2 - 205.  

Ad Ducissam Brabantiae (rescripta)
Full title: Epistola ad ducissam Brabantiae.  Other titles:  Epistola ad Comitissam Flandriae.  It is believed by many this letter was likely written in 1271, not to the “Duchess of Brabant” (as many historians, and even the Leonine editors thought), but rather to Margaret, Countess of Flanders, who had a great interest in the Dominican Order, and likely knew Thomas directly.  On the Government of Jews in Aquinas.  Translated by J. G. Dawson in Aquinas: Selected Political Writings , pp. 84 – 95 (Stacks, BQ 6923 .D27).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 42, pp. 357-378.

Contra Errores Graecorum (recensiones)
Translated by Peter Fehlner in Ending the Byzantine Greek Schism: Containing the 14th c. "Apologia" of Demetrios Kydones for Unity with Rome (Stacks, BXZ 303 .L54 1983). Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 40 A.

De Emptione (rescripta)
Full title: De Emptione et Venditione ad Tempus.  Translated works available in the library: The Pocket Aquinas: selections from the writings of St. Thomas, pp. 223-225.  Edited and translated by Vernon J. Bourke (Stacks, BQ 6832 .P73).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 42 pp. 379-394.

De Mixtione Elementorum ad Magistrum Philippum de Castrocaeli (rescripta)  
Translated works availabe in the library: Aquinas on Matter and Form and the Elements.  Translated by Joseph Bobik (Stacks, B765.T53 D744 1998). Text in Latin can be found in the Parma edition of Opera Omnia, v. 16, p. 353. Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 131-157.

De Motu Cordis ad Magistrum Philippum de Castrocaeli (rescripta) 
Saint Thomas Aquinas on the Movement of the Heart.  Translation is available in the following work: Journal of the History of Medicine, vol. 15, pp. 22–30.  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 91-130.  

De Operationibus Occultis  (rescripta)
Full title: De Operationibus Occultis Naturae ad Quendam Militem Ultramontanum.  Othe titles: De Occultis Operationibus Naturae.  Translated works available in the library:  The letter of Saint Thomas Aquinas, De Occultis Operibus Naturae ad Quemdam Militem Ultramontanum.  Translated by Joseph Bernard McAllister (Stacks, B21 . C36 v. 42).   Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 43, pp. 159-186.

De Regno ad Regem Cypri (rescripta) 
Other titles: De regimine principum; De regimine principum ad Regem Cypri.  On Kingship, to the King of Cyprus.  Translated by Gerald B. Phelan (Stacks, BQ 6857 .R34 1949).  Text in Latin can be found in the Leonine edition of Opera Omnia, vol. 42, pp. 417-471.

Epistola de Modo Studendi
Translated works available int he library: How to Study: being the letter of St. Thomas Aquinas to brother John, De Modo Studendi.  Translated by Victor White.  Contains English and Latin (Stacks, BQ 6858 .E56 E5 1947).