ACM SIGAPL The Association for Computing Machinery's Special Interest Group for Array Programming Languages has some J material.
BrianSchott's YouTube J screen casts: download and install, an introduction via Labs, browsing locales with cobrowse.ijs, and a simple openGL screen robot. Search for "J Programming Language" for more.
Cliff Reiter's Gallery of Fractals, Chaos and Symmetry at Lafayette College.
Donald B. McIntyre's site includes Mastering J Revisited.
Ewart Shaw's page at University of Warwick has a thorough guide to J.
John Howland's pages at Trinity University, San Antonio.
Keith Smillie's J Page at University of Alberta has several excellent introductory articles, including a statistical package written in J.
Ken Gordon has a review A management perspective of the J programming language among other recommended data warehousing and compression articles, some archived.
Martin Neitzel's Juggler's page at Gaertner Datensysteme GbR.
Mstat is a standalone package for statistical analysis, developed by Norman Drinkwater at the McArdle Laboratory for Cancer Research, University of Wisconsin.
Oleg Kobchenko's J Page has several useful J scripts.
Словарь Языка J (archive, PDF). Russian translation of J dictionary. This is a working copy, sublect for discussion and change. Please comment on it here. Periodically (e.g. after J 6.01 is out) it will be assembled into a higher quality releases, made available elsewhere.
Skip Cave's site includes various historical APL/J papers, as well as digital pictures taken at the J2000 conference in Toronto.
LiteratePrograms Examples of J code elaborated in "literate" style.
Rosetta Code Examples of J code for comparison and contrast with other programming languages.
Vector Journal of the British APL Association with many articles on J.
Wikipedia entry J programming language