FlagRSP and its descendents have long been the de facto standard addons that most roleplayes use to "flag" themselves, letting other roleplayers know at a glance not only that they're interested in roleplaying, but also sharing character descriptions and basic information about what sort of roleplaying they prefer. The original FlagRSP is defunct, but the torch has been passed to two worthy (and superior) successors: FlagRSP2 and MyRolePlay. Both use the same methods to communicate with other roleplayers, so you need only choose one and you'll be able to share information with players who use the other (be careful not to use both at the same time, however). Both addons help a lot with the problem of roleplayers being hard to find.
FlagRSP2 and MyRolePlay both give you a space to write things like your character's first or last names, a character title (such as "Priestess of the Dark" or "Wacky Troublemaker"), as well as some description about what your character looks like; and of course they both enable you to see the information other people have written about their characters too. Both have "roleplaying flags," which can tell other people whether you are a "casual" or "fulltime" roleplayer, as well as whether you are in or out of character at any given moment.
FlagRSP2 has a cleaner, more intuitive interface, in my opinion, and it has nice little popup windows for character information which can appear whenever you mouseover or target someone. MyRolePlay only has a popup button which you must click on to see other characters' information, but it also has two separate spaces for physical descriptions and story backgrounds, which FlagRSP2 users sometimes mix up together. MyRolePlay also uses less computer resources on my system, and keeps things running a bit smoother. I often switch between the two since I can't really decide which is best.