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
** Water in solid salt or sugar, forming moist solids
** Hexane in paraffin wax
** Polymers containing plasticizers such as phthalate (liquid) in PVC (solid)
* Solid in solid:
** Steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms 
** Alloys like bronze and many others
** Radium sulfate dissolved in barium sulfate: a true solid solution of Ra in BaSO