I've posted before about FSI-language-courses.com, a site dedicated to disseminating language courses prepared by the United States government (and thus in the public domain).
The site's owner seems to be AWOL, but one user on the site has continued his work of tracking down FSI books and tapes and digitizing them. In this thread on the site's forum, he posts external links to download more than two dozen new books and half a dozen more audio courses, until the site owner reemerges and uploads these materials to the site proper.
The following new materials for Southeast Asian language study are now available:
Audio for the Thai Basic Course Vol. 2 (part 1 -- 98MB and part 2 -- 79MB) -- a PDF of the accompanying book, along with Basic Course 1 PDF + MP3, and Intro to Thai Phonology MP3 are here.
Thai Reference Grammar by Richard B. Noss (10MB PDF) -- also available at SEAlang in DjVu format. This new version is very well done; the quality of the scan is much better, and includes bookmarked sections within the PDF.
Contemporary Cambodian Introduction (42MB) -- grammatical sketch and Basic Course Vol. 1 PDF + MP3 and Basic Course Vol. 2 PDF are here.
And as long as we're in Southeast Asia, last time I didn't post about the Vietnamese materials available:
Vietnamese Basic Course Vol. 1 (PDF + 37 MP3 files)
Vietnamese Basic Course Vol. 2 (PDF)
And also not yet added to the site, but linked in the forum: audio files for Vietnamese Basic Course Vol. 2 (part 1 -- 81MB and part 2 -- 69MB).
The following Cambodian materials are also available directly as PDF scans from the U.S. government at www.eric.ed.gov (note that there is some overlap with FSI-language-courses.com):
Contemporary Cambodian: Introduction (670 pages, 1972)
Contemporary Cambodian: Grammatical Sketch (127 pages, 1972)
Contemporary Cambodian: The Land and the Economy (375 pages, 1973)
Contemporary Cambodian: The Social Institutions (392 pages, 1974)
Contemporary Cambodian: Political Institutions (387 pages, 1974)
The quality of these scans varies between acceptable and poor. Also, these predate the Khmer Rouge, so their factual value is probably quite out of date, unfortunately, but their value as advanced readers for the Khmer language remains.
Cassette tapes were also produced for the advanced books in the Contemporary Cambodian series, and can be purchased from ntis.gov, at a prohibitively expensive price ($120 per volume). These are, as is everything the U.S. government produces, public domain materials. I hope someone will set these free online in the future.