Earth Minimal Language

Complete 220 word second compact language.
http://ebtx.com/lang/eminfrm.htm

Constructed Languages

Links to many conlang sites as well as the author's own conlang.
http://www.geocities.com/Athens/Parthenon/7853/index.html

Phonetic Picture-Writing

Phonetic picture-writing can also be read phonetically. Its ideograms are composed of special letters. Authored by Leonhard Heinzmann.
http://members.aol.com/leonheinz/english-lautbildschrift/lautbildschrift.htm

Universal Language Dictionary Project

An attempt to create a list of concepts, described in English, along with words to express those concepts in several natural and artificial languages.
http://ogden.basic-english.org/dicintro.html

Constructed Languages

An encyclopedia entry describing constructed languages, including sections on auxiliary languages, artistic languages, and logical languages.
http://en.wikipedia.org/wiki/Constructed_languages

The German Interlinguistics Society "Gesellschaft für Interlinguistik"

Review article by Seán Ó Riain. Published in "Language Problems and Language Planning" in 2003.
http://www.benjamins.com/jbp/series/LPLP/27-3/art/0004a.pdf

Studoj pri Interlingvistiko / Studien zur Interlinguistik

Journal article by Wim Jansen that reviews a book on language planning and language policy by Sabine Fiedler and Liu Haitao. The book is on the theoretical and applicational aspects of interlinguistics, esperantology, terminology and lexicography.
http://www.benjamins.com/jbp/series/LPLP/27-3/art/0005a.pdf

Conlangs DE-Cal

A class on constructing languages at the University of California Berkeley.
http://www.livejournal.com/userinfo.bml?user=conlangs_decal