High Quality Content by WIKIPEDIA articles! PlanetMath is a free, collaborative, online mathematics encyclopedia. The emphasis is on rigour, openness, pedagogy, real-time content, interlinked content, and also community of about 24,000 people with various maths interests. Intended to be comprehensive, the project is hosted by the Digital Library Research Lab at Virginia Tech. The site is owned by a US-based nonprofit corporation, "PlanetMath.org, Ltd."