  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.
