The Language Construction Kit;

Create plausible and realistic languages for RPGs, fantasy and science fiction, movies or video games, or international communication... or just learn about how languages work from an unusual, light-hearted perspective. The Language Construction Kit on zompist.com has helped a generation of conlangers to understand and create languages. It's expanded here with coverage of semantics and pragmatics, language families, writing systems, and sample wordlists, as well as an annotated sample grammar.