Constructed

Bibliography of Planned Languages (Excluding Esperanto)
List of books on interlinguistics, Volapük, Ido, Ro, Occidental, Novial, Interlingua, Glosa, modified English and Latin, pasigraphies, number languages, and other planned languages.
http://www.rickharrison.com/language/bibliography.html

Conlang Free City
This Conlang Wiki is devoted to the topics of ConLangs and ConCultures. This is an online resource and communnity about Conlangs that you can edit.
http://conlang.wikia.com/wiki/Main_Page

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

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

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

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

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. [PDF]
http://esperantic.org/librejo/dbstudoj/index.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