Simulators are software programs that have grown in popularity among design groups in recent years. Once an ISA is finalized, and the basics of the datapath are mapped out (especially the timing and delays), a simulator can be a very valuable project to work on.
A simulator allows software for your new microprocessor to be tested on a separate computer. This means that people can write and test software for your new processor even before you have finished designing it!
Simulators have lead to a fascinating new realm of productivity known as hardware software co-design.