Earth Minimal language - Are you feeling terse today? It may be hard to get any more precise than in an "Earth Minimal Language" expression. Check out this incredible language from EBTX. - http://www.ebtx.com
Model Languages - Free open source LangMaker software for Windows to help design consistent languages for all kinds of settings. Build a vocabulary from scratch or extrapolate from real examples. - http://www.langmaker.com/
The Language Construction Kit - Intended for anyone who wants to create artificial languages -- for a fantasy or an alien world, as a hobby, as an interlanguage. It presents linguistically sound methods for creating naturalistic languages -- which can be reversed to create non-naturali - http://www.zompist.com/kit.html
Constructed Human Languages - Comprehensive guide to various invented and artificial languages. Includes links to numerous invented language projects on the web. - http://www.quetzal.com/conlang.html