Instrumentum Europe
Bibliography on crafts and manufacturedproducts in antiquity (since 1996).
http://www.instrumentum-europe.org/bibliography.html

Dyabola : subject catalogues of publications/images on the ancient world(Deutschesarchäologisches Institut and major Germanlibraries)
The ultimatebibliographictool for archaeologists. Quiteexpensive user licence, but researchers are allowed to browsedyabola free-of-charge once per year : use the « get a demo-keynow » link on the « web-access » section).
http://www.dyabola.de/

Library of Congress, Washington D.C., U.S.A.
The world’slargestlibrarywith excellent researchdatabases (books only)
www.loc.gov

Persée : programme de publication électronique de revues scientifiques en sciences humaines et sociales.
Lots of download-free articles of major French publications.
http://www.persee.fr/

Academia
The leadingresearch-sharing academicwebsite, wheremanyarcheologists propose their articles in download-free pdfs.
www.academia.edu

Internet archive
Thousands of pre-1930 books (no more under copyright) in online digital version and download-free pdfs.
http://archive.org/