Here are more "dirty" tricks left out...
#10. Pillage road in neutral territory - this won't cause war and you break a city connection.
#11. Pillage lux of a city state to cause another player to lose happiness. Of course take the worker so it won't be able to repair the lux.
#12. Declare war, then immediately make peace with a player. This happens a lot when people have a unit stuck by expanded boarders. Now that they cannot declare war on you for a bit, exploit the forced peace... such as attack other player's ally so they cannot help out. Or pillage road in neutral territory and sit unit on the road so it can't be repaired... or use you units to block his settler/units, etc... lots of creative ways to exploit this.
#13 starving coastal cities - declare war, and keep ship out of ranged unit attack, but now their coastal resources cannot be worked.
#14 The Big City State steal - buy up opponent's city states and declare war before they are able to buy them back.
#15 Plant a city close to opponent's capital, then citadel right to his city - if he has been at peace all game and you were at war saving generals, you can chain citadels and he won't be able to citadel back for many turns... during which time he loses many of his good tiles and units next to his cap lose hit points every turn... with roads on these citadels, multiple mounted units can strike the cap every turn and retreat.
#16 - Cargo ship weakness - works best when you are the first person to Astronomy. Use fast caravel to pillage all sea trade routes.
Some great tricks. In learning times i have been stroke by some of them. Especially the #14. Today, i will DoW a player(often the richest) without attacking him just because i don't want to have some cs stolen while i'm at war with another guy(and know that i will need extra happiness when cities will fall).