FTP (F2P, as I usually abbreviate it) games have always been appealing to me, considering I don't have access to a large sum of money. I tried Maplestory for a few weeks before I gave up on it. Moved on to Combat Arms, which was a great time sink. Played it for just about 5 years now (started June 2008). Albeit yes there are hackers, and the guns are basically pay-to-win, it's still somewhat enjoyable, even to this day.
Nexon aside, I found osu! in 2010. It was a pretty good game, and I put a lot of hours into that too. I took such a long hiatus from osu!, though, and that led to me making a second account because I completely forgot my account details... Some F2P games I've found more recently include Warframe and Blacklight: Retribution. Certainly good shooters, both of them.