It's a really interactive game, I've made tons of friends online and the leveling system is quite fun and challenging. Although it's a 'free' game (which they say in other games...) you have an option to buy 'NX' which is basically upgrades to customize your character's appearance and etc... Overall this game is pretty fun, but if you're a hard core gamer and you wanna lvl fast then you gotta buy nx.. I would recommend this game to a more younger audience such as 10-18? A lot of my friends who are like 21 play it, but to be playing all the way to age you have to start off young.
I'm sure you've heard of WoW and probably curse it for it's over priced subscriptions. And I know this doesn't classify under free, but if Maplestory is too kid-ish for you and you want some of that hard core gaming then I would.. Sadly.. recommend WoW. (World of Warcraft) In the future when I have a job or something that I could make money off I'll probably join WoW but for now I'm just a broke 14 year old xD.
Maplestory is only one of the MMORPGs that Nexon has made. Nexon is the producer of a few MMORPGs if you wanna check out the other ones, then heres the site.
Sorry I haven't really checked out their MMORPGs but I think I can overview them for you:
I've played this game at my friend's house for a bit, but It kinda looks like an anime-style adventure fighting game. If I'm correct.
This game hasn't released yet but I've visited the site and checked out the plot line as well as the trailer. As far as I can see this game kinda looks like a rip off of WoW but with it's own unique style. (note: they classify this as an 'Brutal' MMO, but it's pretty much an MMO)
So the summary for these four games are:
Maplestory: 2D MMORPG, good for talking with friends, challenging and fun leveling system. (FREE)
Mabinogi: Anime-Style Adventure Fighting game. (FREE)
Vindictus: Rip off of WoW (no offense), with it's own unique gaming style. (FREE)
WoW: An Extremely expensive tempting MMORPG, one of the greatest online games in the world. I think World of Warcraft is a little self explanatory; I don't need to get in depth. (DEFINITELY NOT FREE)
Hope it helps,
P.S. I personally hate runescape for it's crappy Lego graphics and it's extremely limited variety and effort they put into that game.