Arguably the most popular game being run at Quincon this year is the Pathfinder RPG. It started off as an evolution of Dungeons and Dragons edition 3.5, but has maybe become more popular than 3rd edition D&D ever was. In addition to a RPG line, they have an expansive line of adventures, fiction, miniatures, setting material, and an organized play system. Quincon has many of these organized play modules on the schedule if you are interested in trying it.
To me, the most interesting thing about Pathfinder is the history of the game. When Dungeons and Dragons 4th edition came out, lots of people who enjoyed the 3rd edition were quite put out. The publisher of Pathfinder, Paizo, decided to stick with the 3rd edition basics and make their own game with a few tweaks. Thus Pathfinder was born. It was in open playtest for over a year, where they gave away free copies of the beta version and took in all the comments and fine tuned the game. This had the side effect of flooding the market with Pathfinder interested roleplayers.
The end result is a very well run, very well supported roleplaying game with a pre-existing fan base. It has really taken off, and I have heard rumors that it is more popular that the 4th edition of Dungeons and Dragons.
Come to Quincon for all the Pathfinder RPG you would ever want!
p.s. Not to be outdone, a 5th edition of Dungeons and Dragons is on the horizon. Hopefully someone will run a game of that with their free beta rules.
Here is the link to the publishers website: Pathfinder