Plurking is basically a social network tool. I don't know how familiar you are with Facebook, but it's essentially a horizontal version. You (or anyone) can make a plurk, aka a post about whatever, and people comment, sort of like an AIM or FB conversation.

People have argued that it works better than AIM given that you don't have to deal with people being online at the same time, such as different timezones and the like. You can do things like private plurk or ping someone (notify them) within a plurk so they know where to direct their attention to.

Personally I use my plurk for non RP stuff, so I only have a limited number of friends, but it's handy for plotting because of the above reasons.