  We describe a fully broadband approach for electron spin resonance (ESR)
experiments where it is possible to not only tune the magnetic field but also
the frequency continuously over wide ranges. Here a metallic coplanar
transmission line acts as compact and versatile microwave probe that can easily
be implemented in different cryogenic setups. We perform ESR measurements at
frequencies between 0.1 and 67 GHz and at temperatures between 50 mK and room
temperature. Three different types of samples (Cr3+ ions in ruby, organic
radicals of the nitronyl-nitroxide family, and the doped semiconductor Si:P)
represent different possible fields of application for the technique. We
demonstrate that an extremely large phase space in temperature, magnetic field,
and frequency for ESR measurements, substantially exceeding the range of
conventional ESR setups, is accessible with metallic coplanar lines.
