The National Library of Virtual Manipulatives is a project created by Utah State University with the help of the National Science Foundation. Here is a description from the website:
The National Library of Virtual Manipulatives (NLVM) is an NSF supported project that began in 1999 to develop a library of uniquely interactive, web-based virtual manipulatives or concept tutorials, mostly in the form of Java applets, for mathematics instruction (K-12 emphasis). The project includes dissemination and extensive internal and external evaluation.
This is a terrific resource for teaching mathematics. The money image on the left represents one manipulative where students drag bills and coins into a rectangle to make a listed amount. These virtual manipulatives are divided by grade level (preK-2, 3-5, 6-8, 9-12) and by strand (number and operations, algebra, geometry, measurement, and data analysis and probability). Find this website and bookmark it. Students will quickly learn to work on these independently and be provided with an engaging way to build math skills.