Presented in this paper are experimentally measured relative complex permittivities as a function of frequency for a representative range of ionic liquids, comprising 64 imidazolium, choline, phosphonium, ammonium, pyrrolidinium, pyridinium and piperidinium cations with a variety of anions. The measurement was conducted using a coaxial probe technique at room temperature from almost DC (0 Hz) to 18 GHz with a high level of accuracy. The real parts of the complex permittivities have been summarized in table format with both the real and imaginary parts presented as individual spectra for each ionic liquid. It is found that an increase in the molecular weight results in a decrease in overall permittivity and this is true over multiple series of ionic liquids. Additives such as water are shown to have a profound effect on the complex permittivity. The measured data is useful for a wide range of applications, such as electronic devices, chemical synthesis and materials.