As every scholar and university student knows, or should know, not all online content is of academic value. That said: there is a lot of very useful material stored on the internet, if you know where to look. From the very start of my research project I have collected digital versions of the primary sources I work on: to my amazement even the most obscure publications by the most forgotten authors have been digitized and put online, waiting for you to leaf through or even download them at will. In addition, there are digital versions of the most valuable, thoroughly academic printed compendia available on the internet. In a library their severe-looking volumes typically occupy several stretches of shelf; their online incarnations welcome you with a friendly search-screen.
So where do you find digital full-text versions of 18th century architectural treatises, for instance? On Google books of course, although it has its little idiosyncrasies. When you need a French treatise, for some reason you will have more luck on http://books.google.fr then on http://books.google.com. And since there is no dearth of material, you have to be quite specific in you search terms and refinements – unless the author you are looking for is called Séroux d’Agincourt, or some other distinctive name.
Still, not everything is on Google books. And even if it is, you might have trouble finding it. So during the past two years I have developed some back-up plans. If it is a German text I am looking for, I pay a visit to the digital library of the Ruprecht-Karls Universität of Heidelberg; for French texts I go to www.gallica.fr, that wonderful digital repository of the Bibliotheque Nationale de France; and for English ones I search the database of ECCO – the Eighteenth Century Collections Online – which I can access via my Leiden University account. If none of them yields anything, www.europeana.eu is my last resort. This is a website that does not offer any digitized materials itself, but points you to a variety of university library websites that do.
A special case is presented by French journals from the 18th century. When I need one of those, I head straight for the Gazetier Universel. This brilliant website lists practically all issues of every journal ever printed in 18th century France, with a link to digital full-text versions. What’s more, it entirely reproduces Jean Sgard’s monumental Dictionnaire des Journaux (1991), which provides detailed histories of the journals listed.
In terms of compendia, the first that comes to mind is the Dictionary of the History of Ideas (the third volume of which sports two useful entries on primitivism, by the way), digitized in its entirety by the University of Virginia. Close second is the online Catholic Encyclopedia: very useful if you want information on, for example, authors and public figures affiliated with the church.
Another category of compendia susceptible to digitization seems to be that of biographical dictionaries: so far I have found online versions of an English one – the Oxford Dictionary of National Biography, again available via my Leiden account; a German one via www.deutsche-biographie.de – which offers both the Neue Deutsche Biographie (1953-now) and the Allgemeine Deutsche Biographie (1875-1912) ; and that Italian masterpiece in progress, the Dizionario Biografico degli Italiani, accompanied by some minor biographical compendia at www.treccani.it. To my regret, I have yet to find a French equivalent. Please leave a comment if you know of one!
 There is probably a perfectly reasonable explanation for this. If you have one, please leave a comment below!