This interesting and very useful circuit I have found on this site here . It is a variable clock generator ranging from 4MHz up to 56MHz. The ranging can be set with the 4-Dip switches and a 500Ω potentiometer. It really comes handy when you are trying to experiment with with different clocks for you Microcontroller, Microprocessor. It even can generate  higher clock speeds for your FPGA and CPLD projects.