Pauly-Wissowa’s Realencyclopädie (or in its full title, Paulys Realencyclopädie der klassischen Altertumswissenschaft) is an indispensable resource for the study of the ancient world. Curious about a name, place, or person of Greek or Roman antiquity and not sure where to start? If you can read German, PW is a great choice. It certainly has helped me move forward on many occasions. Many articles from PW are old enough that they are no longer under copyright, and may be digitally transcribed and made available, open-access, online. AWOL has just updated its post with links to a wiki of digitized, open-access versions of the articles in PW, here.
The digitization of PW, whose volumes number in the dozens, is ongoing, so the wiki is hardly comprehensive. The digitization team is happy to accommodate requests for digitizations of articles. Requests for digitalization can be made via the wiki here, which also describes the conditions under which the team may legally digitize the articles.