Bowline; Round Turn And Two Half Hitches; Tug Boat Hitch; Anchor Bend; Constrictor Knot/Double Constrictor Knot; Rolling Hitch/Midshipmans' Hitch; Truckers' Hitch; Clove Hitch; Buntline Hitch are a few good ones IMHO. Depends on what you're trying to do: rig a tarp, tie a fishhook, sail a boat, haul some firewood, hang a violator of the local "law". You tie a rope or cord for stopping (e.g. Figure Eight), to something else (Round Turn And Two Half Hitches, or the Tug Boat Hitch - which won't jam); to itself, e.g. as guy rope (Midshipmans' Hitch); coil and secure your rope (Buntline Hitch); lashing (Truckers' Hitch). For the violator in the above scenarios a Hangmans' Noose is a goody. There are also decorative knots for things such as belts, knife lanyards, and so on.
Take a look at Ashley's Book of Knots - some 3500 knots and lashings or so plus loads of historical background. And a great diversion.