Here’s a set of links to the complete works of John Calvin in their original Latin and French. They can be downloaded as PDFs from the Université de Genève. The whole set is also available in searchable (but not downloadable) text from The Korea Institute for Calvin Theological Studies.
Ioannis Calvini opera quae supersunt omnia. Edited by G. Baum, E. Cunitz, and E. Reuss. 59 vols. Corpus Reformatorum 29–87. Brunswick: C.A. Schwetschke, 1863–1900.
Of course, these are all out of copyright, so they are legally accessible in PDF or other formats from GoogleBooks and archive.org, where available.
2. Institutio religionis christianae. Editio ultima 1559 (GoogleBooks)
3. Institution de la religion chrétienne 1560, t. 1 (GoogleBooks)
4. Institution de la religion chrétienne 1560, t. 2 (GoogleBooks)
6. Tractatus theologici minores, t. 2 (GoogleBooks)
7. Tractatus theologici minores, t. 3 (GoogleBooks)
8. Tractatus theologici minores, t. 4 (GoogleBooks)
10/1. Tractatuum theologicorum. Appendix (GoogleBooks)
One should also note that a new, critical edition of these works is being released through Librairie Droz in Geneva.
Ioannis Calvini opera omnia denuo recognita et adnotatione critica instructa notisque illustrata. Geneva: Librairie Droz, 1992–.