Erasmus published five editions of his Latin-Greek New Testament, with annotations, in 1516, 1519, 1522, 1527, and 1535. Gradually, all these editions have become available online. There were also Latin-only editions (not listed here).
The Swiss www.e-rara.ch has Erasmus’ Novum Instrumentum omne of 1516, with the Annotationes (actually one of the copies of the Universtätsbibliothek Basel, shelf mark BibG B 3).
Navigation is easy, and the site’s table of contents is extensive (though not as exhaustive as to contain each NT book or even chapter). The quality of the images is good. From the contents page, one can download PDF files of parts of the book. One can also download a PDF of the entire book.
There is another copy in the digital collections of the Bayerische Staatsbibliothek (Staatliche Bibliothek, Regensburg, shelf mark 999/2Script.238).
There are also GB versions of the 1516 edition: Österreichische Nationalbibliothek, shelf mark 4.D.10; Universiteitsbibliotheek Gent, shelf mark BIB.ACC.015856).
Yet another, interesting copy is found in the Universitäts- und Landesbibliothek Düsseldorf (shelf mark ULBD DUE 01 α); it is from the collection of (first owner) Johannes Cincinnius von Lippstadt, who even noted on which day he bought the book, and how much he paid for it (see f. aaa 1v), and who made extensive use of it.
Erasmus’ second edition of 1519, with the Annotationes, is available as well at e-rara (Universitätsbibliothek Basel, shelf mark FG V 40-41).
GB has the text volume of Universiteitsbibliotheek Gent (shelf mark BHSL.RES.1676).
This second edition of 1519 is also available at the University of Jena; it can also be downloaded as PDF.
It is further possible to make links to specific page views, e.g. Erasmus’ conjecture φθονεῖτε at Jas 4:2, for example, or his annotation on the reading:
Erasmus’ third edition of 1522, with the Annotationes, is available as well at e-rara (Universitätsbibliothek Basel, shelf mark FG V 42-42a).
GB has the annotations volume of Universiteitsbiblioteek Gent (shelf mark BIB.ACC.017827).
Erasmus’ fourth edition of 1527, with the Annotationes, is available as well at e-rara (Universitätsbibliothek Basel, shelf mark FG V 43-43a).
The 1527 volume with the Annotationes is available in the Württembergische Landesbibliothek Stuttgart.
Erasmus’ fifth edition of 1535, with the Annotationes, is available as well at e-rara (Universitätsbibliothek Basel, shelf mark UBH FG V 44:1-2).
The 1535 edition can also be found at the Erasmus Centre for Early Modern Studies, but only the New Testament text, not the Annotationes. Previously the Centre used the DjVU format, but since 2014 it provides PDFs instead.
Other copies can be found at GB: Österreichische Nationalbibliothek (shelf mark 4.O.18: Text and Annotationes); Universiteitsbibliotheek Gent (shelf mark BIB.TH.004088; in one volume).
[12 January 2015: added more 1516 editions and supplied shelf marks for these; updated information on the 1535 copy at ECEMS; updated the page view links to the Jena copy of the 1519 edition; added GB/ONB copies of the 1535 edition.]
[1 Februari 2017: added the e-rara 1519 and 1535 editions; added some more shelf marks; updated URLs to DOI]
[13 May 2017: added some Gent volumes: the 1516 edition; the 1519 text volume; the 1522 annotations volume; the 1535 edition.]