MusiXTeX
Created by Daniel Taupin, Ross Mitchell, and Andreas Egler, the package offers a set of macros and fonts that enables TeX to typeset music.
PSTricks
Access PostScript capabilities inside (La)TeX, by Timothy van Zandt. Site contains documentations, bugs list, and examples of application.
PSTricks.de
Unofficial FAQ with lots of examples, resources, and information about converting to PDF.
SIAM: Tex Resources
Packages for SIAM's journals, books, and conference publications supporting LaTeX, Plain TeX and AMS-TeX.
Springer Journals TeX and LaTeX Packages
Includes packages for Acta Informatica, Archive for Mathematical Logic, Distributed Computing, Inventiones mathematicae, manuscripta mathematica, Mathematical Programming, Mathematische Annalen, Mathematische Zeitschrift, Numerische Mathematik, Probability Theory and Related Fields.
Typesetting Critical Editions with LaTeX and ledma
Ledmac is a package for typesetting critical editions. It is a LaTeX port, and extension, of the plain TeX EDMAC, TABMAC and EDSTANZA macros.
Xy-pic
A free package for typesetting graphs and diagrams in LaTeX, AMS-TeX and Plain TeX using the principle of logical composition of visual components. Presentation, samples and download.