This is an NSF supported project that began in 1999 to develop a library of interactive, web-based virtual manipulatives or concept tutorials, mostly in the form of Java applets, for mathematics instruction (K-12). Ultimately the project aims to make all materials available at several sources on the Internet, creating a national library from which teachers may freely draw to enrich their mathematics lessons. The materials will also be of importance for the mathematical training of both in-service and pre-service elementary teachers. They are particularly useful for use with interactive whiteboards.