Gas
If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air (oxygen and other gases dissolved in nitrogen).
Sodium chloride (table salt) or any other salt in water, which forms an electrolyte: When dissolving, salt dissociates into ions.
Counterexamples are provided by liquid mixtures that are not homogeneous: colloids, suspensions, emulsions are not considered solutions.
Solid
If the solvent is a solid, then gases, liquids and solids can be dissolved.
Gas in solids:
Hydrogen dissolves rather well in metals, especially in palladium; this is studied as a means of hydrogen storage.
Liquid in solid:
Mercury in gold, forming an amalgam
Hexane in paraffin wax
Solid in solid:
Steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms.
Alloys like bronze and many others.
Polymers containing plasticizers
