Diplomacy was a board game, and has a long history as a play-by-mail, then play-by-email, then several persistent website, and webdip (originally phpdip?) came along.
There are several variants, different maps, different chat rules, and different units. But the classic board is probably still the most popular.
It is essentially a game of building trust and knowing when/if to betray that trust. One half tactical strategy, the other diplomatic skill. To survive tue early game you must make allied, to solo the board you must betray them (or push them into betraying you...).
The rules are comparatively simple and intuitive, but unlike most other wargames, and the importance of working with other is right there in the name of the game (Diplomacy). As such it is unlike most other games. Modern team games/co-op vs games/multiplayer strategy games tend to favour fixed alliances when everyone on your side is working towards the same goal. In diplomacy goals can change as necessity and circumstance force former enemies to get each other's back and fight off a larger foe/alliance; only to stab each other again when the time feels right.
Webdiplomacy is an open source project and has several sister sites, vdiplomacy.net being the home of many variants, and several language specific sites catering to non-english speaking groups.
-orathaic, ex-mod, ex-tournament director, ex-variant designer, current forum denizen.