We report new results on the electron spin resonance of single walled carbon nanotubes, peapods and double walled carbon nanotubes. A small ESR line is present in well purified samples and its temperature dependence reveals a striking 'super-Curie' paramagnetic rise. In addition, we identify a narrower line in peapod and DWNT samples, which displays similar behavior. The linewidth of both inner and outer species are metallic and the outer tube's resonance broadens upon growth of the inner tubes. We observe a sudden decrease of relaxation rate below 20 K, in accordance with the opening of a spin-gap.