Diplomacy is a strategic board game created by Allan B. Calhamer in 1954 and released commercially in 1959. It's main features from other board games are it's negotiation phases, where players talk to each other, forming alliances and other deals, and the complete lack of luck in the resolution of combat in the game. It has a wide player-base both online and in e-mail and even postal games.
Check the link below for detailed information on the game itself, including the full text of Richard Sharp's book on the game, The Game of Diplomacy.
http://www.diplomacy-archive.com/home.htm
For the Modern Powers game being run on this site, check here - https://phuks.co/s/ModernPowers/hot
This looks like a very interesting variant, as it abides the standard rules, but adds an extra layer of currency speculation.
Many different strategies thinkable, such as buying-and-holding a strong power like France and leading it like a regular Dippy nation, going for underappreciated weak powers like Italy for guaranteed influence on the physical map, trying to manage 2 or more nations while fending off hostile take-overs, or even going for pure currency speculation without actual Dippy orders.
It should also work with a weird number of players, as it isn't required to actually control a nation. As long as you aren't bankrupt, you are in the game.