A large variety of inorganic radicals are stable and in fact isolable. Examples include most first-row transition metal complexes.
With regard to main group radicals, the most abundant radical in the universe is also the most abundant chemical in the universe, H. Most main group radicals are not however isolable, despite their intrinsic stability. Hydrogen radicals for example combine eagerly to form H. Nitric oxide (NO) is well known example of an isolable inorganic radical. Fremy's salt (Potassium nitrosodisulfonate, (KSO)NO) is a related example. Many thiazyl radicals are known, despite limited extent of π resonance stabilization.
Many radicals can be envisioned as the products of breaking of covalent bonds by homolysis. The homolytic bond dissociation energies, usually abbreviated as "ΔH°" are a measure of bond strength. Splitting H into 2 H, for example, requires a ΔH° of +435 kJ/mol, while splitting Cl into two Cl requires a ΔH° of +243 kJ/mol. For weak bonds, homolysis can be induced thermally. Strong bonds require high energy photons or even flames to induce homolysis.