The history of the publication of Leibniz's writings is a long and complicated one. The following list is not intended to be exhaustive. For further details, see Roger Ariew, "G.W. Leibniz, life and works," in The Cambridge Companion to Leibniz, ed. Nicholas Jolley (New York, 1995). A comprehensive bibliography can be found in Emile Ravier, Bibliographie des Oeuvres de Leibniz (Paris, 1937; repr. Hildesheim: Olms, 1966).
Recently a wonderful new resource has become available from the Bibliothèque Nationale in Paris. Now many of the editions listed below can be accessed online via the BN’s Gallica website. Highlighted titles lead directly to the appropriate text. The site itself, which contains a huge collection of other published works, can be accessed here (requires Adobe Acrobat).
Gottfried Wilhelm Leibniz: Sämtliche Schriften und Briefe. Ed. Preussischen (later: Deutsche) Akademie der Wissenschaften zu Berlin. Darmstadt/Leipzig/Berlin: Akademie-Verlag, 1923- .
The Academy edition will eventually include all of Leibniz's writings in a projected 120+ volumes. Individual volumes are being published chronologically in seven series:
Series I: General, Political and Historical Correspondence (15 volumes published,
covering the period through September 1698)
Series II: Philosophical Correspondence (one volume published, covering the
period through 1685; series inactive)
Series III: Mathematical, Scientific and Technical Correspondence (four volumes
published, covering the period through 1690)
Series IV: Political Writings (three volumes published, covering the period
through 1689)
Series V: Historical Writings (no volumes published; series inactive)
Series VI: Philosophical Writings (four volumes published covering the period
through 1690, plus volume 6, which contains the Nouveaux Essais)
Series VII: Mathematical, Scientific and Technical Writings (two volumes
published, covering the period through 1676)
Textes inédits d'après des manuscrits de la Bibliothèque provinciale d'Hanovre. Ed. Gaston Grua. 2 vols. Paris, 1948; repr. New York: Garland, 1985. [Important edition that made available many new writings on theology and ethics]
Leibnizens Nachgelassene Schriften: Physikalischen, Mechanischen und Technischen Inhalts. Ed. E. Gerland. Leipzig, 1906; repr. New York: Johnson Reprint Corp., 1973.
Opuscules et fragments inédits de Leibniz. Ed. L. Couturat. Paris, 1903; repr. Hildesheim: Georg Olms, 1961. [Landmark edition that brought to light many important logical and metaphysical writings]
Der Briefwechsel von Gottfried Wilhelm Leibniz mit Mathematikern. Ed. C.I. Gerhardt. Berlin, 1899; repr. Hildesheim: Georg Olms, 1962.
Der Briefwechsel des Gottfried Wilhelm Leibniz in der Königlichen Öffentlichen Bibliothek zu Hannover. Ed. E. Bodemann. Hannover, 1895; repr. Hildesheim: Georg Olms, 1966. [Complete index of Leibniz's correspondences]
Mittheilungen aus Leibnizens ungedruckten Schriften. Ed. Georg Mollat. Leipzig, 1893. [Contains some juridical writings not available elsewhere]
Die Leibniz-Handschriften der Königlichen Öffentlichen Bibliothek zu Hannover. Ed. E. Bodemann. Hannover, 1889; repr. Hildesheim: Georg Olms, 1966. [Complete index of Leibniz's manuscripts, with some excerpts from texts not available elsewhere]
Die philosophischen Schriften von Gottfried Wilhelm Leibniz. Ed. C.I. Gerhardt. 7 vols. Berlin, 1875-90; repr. Hildesheim: Georg Olms, 1978. [Remains the most useful scholarly edition of Leibniz's philosophical writings]
Die Werke von Leibniz. Reihe 1: Historisch-politische und staatswissenschaftliche Schriften. Ed. O. Klopp. 11 vols. Hannover, 1864-84.
Briefwechsel zwischen Leibniz und Christian Wolff. Ed. C.I. Gerhardt. Halle, 1860; repr. Hildesheim: Georg Olms, 1963.
Nouvelles lettres et opuscules inédits de Leibniz. Ed. L.A. Foucher de Careil. Paris, 1857; repr. Hildesheim: Georg Olms, 1971.
Lettres et opuscules inédits de Leibniz. Ed. L.A. Foucher de Careil. Paris, 1854; repr. Hildesheim: Georg Olms, 1975.
Leibniz' Mathematische Schriften. Ed. C.I. Gerhardt. 7 vols. Berlin, 1849-63. [Remains an essential resource for Leibniz's writings on mathematics and dynamics]
Leibnizens gesammelte Werke. Ed. G.H. Pertz. 4 vols. Hannover, 1843-47.
God. Guil. Leibnitii Opera philosophica quae exstant Latina, Gallica, Germanica omnia. Ed. J.E. Erdmann. Berlin, 1839-40; repr. Aalen: Scientia Verlag, 1974.
Gothofredi Guillelmi Leibnitii Opera Omnia. Nunc primum collecta.... Ed. L. Dutens. 6 vols. Geneva, 1768; repr. Hildesheim: Georg Olms, 1989. [Comprehensive edition, spanning the breadth of Leibniz's interests; remains a valuable resource]
Oeuvres philosophiques latines et françoises de feu M. Leibniz. Ed. R.E. Raspe. 7 vols. Amsterdam, 1765. [First major edition of Leibniz's philosophical writings; first publication of the Nouveaux Essais]
De Summa Rerum: Metaphysical Papers 1675-1676. Ed. and trans. G.H.R. Parkinson. New Haven: Yale University Press, 1992. [Translation of Leibniz's Paris notes, based on the critical text of the Academy edition; first in a projected multivolume series of English translations]
Philosophical Essays. Ed. and trans. Roger Ariew and Daniel Garber. Indianapolis: Hackett, 1989. [Most useful student edition of Leibniz's writings]
Political Writings. Ed. and trans. Patrick Riley. Cambridge: Cambridge University Press, 2nd ed. 1988.
New Essays on Human Understanding. Ed. and trans. Peter Remnant and Jonathan Bennett. Cambridge: Cambridge University Press, 1981. [Translation of Leibniz's unpublished, book-length response to John Locke's Essay Concerning Human Understanding; contains valuable appendix of notes on related topics]
Philosophical Writings. Ed. by G.H.R. Parkinson; trans. Mary Morris and G.H.R. Parkinson. London: Dent, 1973. [Student edition; contains several important essays not available in other translations]
Philosophical Papers and Letters. Ed. and trans. Leroy E. Loemker. Dordrecht: D. Reidel, 2nd ed. 1969. [The most extensive collection of Leibniz's writings in English]
The Leibniz-Arnauld Correspondence. Trans. H.T. Mason. Manchester: Manchester University Press, 1967.
Logical Papers. Ed. and trans. G.H.R. Parkinson Oxford: Clarendon Press, 1966. [Best source in English for Leibniz's logical writings]
Monadology and Other Philosophical Essays. Ed. and trans. P. and A.M. Schrecker. Indianapolis: Bobbs-Merrill, 1965. [Contains a translation of Causa Dei, Leibniz's Latin summary of the Theodicy]
Theodicy: Essays on the Goodness of God, the Freedom of Man, and the Origin of Evil. Ed. Austin Farrar; trans. E.M. Huggard. La Salle, Il: Open Court, 1952. [Translation of the only philosophical book published by Leibniz]
G.W. Leibniz, Selections. Ed. and trans. Philip P. Wiener. New
York: Scribner's, 1951.