Play Diplomacy Online

I tried out conversions in Buffy, as a variable third party type (so only a few players could be converted, and could be converted more than once). I wasn't a huge fan of it - it served my flavorful purposes, and don't think it ruined the game, but I would caution against informing susceptible people that they might change alignments- it produces town players that aren't fully behind the town.

The downside to that, of course, is that a converted player's playstyle might change dramatically, and lead to them being caught. It's a tricky thing to work out.