GNU Chess
The IQ6 test suite (a collection of chess problems from Livshits's book Test Your Chess IQ) indicates that on a single core of an Intel Core 2 Duo CPU GNU Chess performs at the senior master/weak international master strength of 2500+ on the Elo rating system.
It is often used in conjunction with a GUI program such as XBoard or glChess, where it is included as the default engine. Initial versions of XBoard's Chess Engine Communication Protocol were based on GNU Chess's command-line interface. Version 6 also supports the Universal Chess Interface (UCI).