Casio Micro Mini Calculator
The Casio Micro Mini is a calculator used by Egon to figure out the interest rate on Ray's house, in the scene where Peter, Ray, and Egon sell Ray's house to start up Ghostbusters.
Although it has been identified as the Casio Micro Mini, there are several different models and finishes, of which it could be. As seen here It could either be the M-800 (with the smooth silver finish, not the speckled or the holographic ones) or the M-810. The most notable differences between the two models is the location of the on/off switch. The M-800 has the on/off switch on the front and the M-810 has the switch on the side.