|Drawing Techniques: a Guide|
Daniel V. Thompson, The Practice of Tempera Painting (online edition) -- Methods of Drawing
The Drawing Tools Forum
The Waterbased Forum
Notebook, 1993-, M a t e r i a l s & M e t h o d s
The American Institute for Conservation of Historic & Artistic Works
Books and Paper Group includes link to BPG Annual (1993-95), which contains many informative articles on technical subjects.
The Conservation Link
CoOl (Conservation on Line) is host to several of the most important resources and organizations for paper conservation, including the AIC, The Institute of Paper Conservation (UK), ICOM, and the European Confederation of Conservator-Restorers' Organisations.
The Iron-gall Ink Corrosion Web Site is an exhaustive source of information about iron-gall ink and its destructive ways. Don't miss "The Unrivalled Ink-Corrosion Show of Horrors!"
The Drawing Site (formerly Michael Miller Lucy Vivante Fine Arts)
Istituto per l'Arte e il Restauro Palazzo Spinelli
Progetto Argos (Project on multilingual conservation terminology) 
Other glossaries of art terms
Raman Spectroscopic Library of Natural and Synthetic Pigments (pre- ~1850 AD)