Arguably, no written work, outside of Scripture itself, has had a more formative influence on Reformed doctrine and practice than Calvin's Institutes. The link above takes you to the most readily available online edtion of the Institutes (Beveridge, 1845). Calvin's original Latin version of his Institutes has been translated into English (in full) by four translators. Three of these four translations are in the public domain and are available to download.
Thomas Norton, 1532-1584, was the earliest translator. Six editions of his work are available, but the most complete and perhaps easiest to read copy is that of 1587, the 1599 copy is missing a few pages of the Scripture index at the end, for example. The other editions are 1574, 1578, 1582, 1599, 1634.
There are two editions of the translation of John Allen, 1771-1839, available. A three volume set published in 1816, originally in the library of Professor Samuel Miller of Princeton (complete with his signature on the title pages!). Volume 1,Volume 2, and Volume 3. There is also a two volume set published in 1813. Volume 1, and Volume 2.
The third translator, and probably the most commonly known one is Henry Beveridge (1799-1863). His work was originally published by the Calvin Translation Society and it is that edition which is available in three volumes. Volume 1,Volume 2, and Volume 3.