In this work, we investigate the metal-free magnetism of triangulene and its analogues (see Figure ) with advanced ab initio methods, i.e., with coupled cluster single-double and perturbative triple (CCSD(T)) and CASSCF/NEVPT2, which take into account both static and dynamic electron correlations. We first discuss in detail triangulene and its dimer, gaining fundamental insight into the magnetic interaction and electron correlation. Additionally, we create a comprehensive dataset of 25 metal-free magnetic systems with non-local spin densities They span a wide range of regimes in magnetic coupling and spin-polarization energy. These systems will serve as reliable references to evaluate the performance of various DFT functionals. Using these reference calculations, we benchmark 22 DFT functionals, covering generalized gradient approximations (GGAs), meta-GGAs, hybrid GGAs, hybrid-meta GGAs, and range-separated hybrids. Our results show that, while achieving quantitative accuracy is still impossible, the hybrid functionals PBE0, MN15, and M06-2X produce qualitatively correct results with moderate deviations in energy, and are thus suitable starting points for methods such as the GW or the RPA.