Diplomacy is one of my favorite games; however, I haven't been able to play in a while. Diplomacy is like Risk without the dice (and without the luck). Basically there are 7 countries that start out with 3-4 army and navy pieces. Each turn there are 15 minutes to confer with everyone else (planning moves, forming alliances, etc.). Everyone writes down their moves on slips of paper and all moves are made simultaneously with a set procedure for resolving conflicts. Games usually last all day long... it's very difficult to find 7 people who can commit to that time.
I win a lot of Diplomacy games through my superior strategy. A key tactic of mine is the preemptive doublecross where I attack an ally. When the victim whines about my backstab I point out that he was about to backstab me 2-4 moves down the line, so it was only fair that I attack as preemptive revenge. George W Bush is a student of my Diplomacy tactics.
I've played a few games online (including a variant with a map of the whole world instead of just Europe), but I didn't do very well. It's much more fun in person.