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Electron spin resonance (ESR) is a powerful tool for characterizing and manipulating spin systems, but commercial ESR spectrometers can be expensive and designed to work in narrow frequency bands. This work presents an inexpensive spectrometer that, when coupled with easy-to-design resonators, enables ESR over a broad frequency range, including at frequencies outside the standard bands. It can operate at either a single frequency or at two frequencies simultaneously. The spectrometer is built from off-the-shelf parts and controlled by a field programmable gate array (FPGA), and new capabilities can be easily added by reconfiguring the FPGA and adding or swapping components. We demonstrate the capabilities of the spectrometer using the molecular nanomagnet Cr$_7$Mn., Comment: 9 pages, 7 figures