I've lived here for two and a half years, here's my experience with the breakdown.
Taekwondo has pretty much become a playground/really expensive baby sitter. Few adults practice it, and most kids have black belts in it. It's a great money maker, and coaches often realize when they have a kid who is special. They'll try to push them into competing at a higher level if they aren't extremely bright. They might go on to compete at the high school varsity or even university level. Basically, they're professional, but you'll never see these people at a local dojang. They're 100% supported by whoever they're fighting for.
ITF Taekwondo is present in South Korea. They have like 4 dojang total. This is done mainly by adults. I'm under the impression that a lot of foreigners end up there as well, because ITF has enjoyed much larger success in Europe and parts of the US.
Hapkido is a mix. Some schools are all kids. Some schools are basically fitness and gymnastic centers with a few wrist locks thrown in. They put on great shows, but aren't so huge on technique until you get past chodan. Other schools have a good number of adults in a range from younger to older, and they spar a lot more.
Yudo (Judo) is good. They have a lot of athletic adults, mostly males. In fact, I've never seen a Korean female doing Yudo that wasn't at the international level. It's just not as cheap or widely available as in the US. Korea has a high percentage of dan ranks in relation to the overall number of Judo players.
Kumdo (Kendo) is not as widely available as Haedong Kumdo (I'll cover this next), but it is fairly popular. You must check out their certs though. Otherwise the two can be easily confused. You won't confuse the fact that Kumdo here concentrates on competition just like Kendo.
Haedong Kumdo seems, from my eye, to be more widely available than actual Kumdo. It mixes some fancy iaido with ridiculous (double twirling sword) iaido. They have most of the same cuts as Kumdo/Kendo, but sparring is not the main focus. From what I've been told though, and what I've seen Kumdo either Haedong or real dojang attract a lot of pretty girls. No clue why, but that's the way it looks. So if you're single, check out kumdo here.
Kyokushin Karate is enjoying some popularity, and it mostly done by athletic males from university age to the mid-30s if they're beginners. It's worth checking out, but you need to check out their lineage, because some people teach TKD as Kyokushin to make bank off of the movie Fighter in the Wind. People lying for money? Crazy, I know.
Shotokan Karate is not popular here. I know of exactly one place where they do Shotokan.
Tang Soo Do is semi-popular, but Taekwondo has almost pushed it out. I know Tang Soo Do has it's roots in Shotokan, but they are separate here. I've never been to a Tang Soo Do dojang, nor have I seen their vans driving around. Note: pretty much every dojang has a van for taking the children home after class.
Kuksulwon and Taekkyon, I have barely seen in Korea. Taekkyon, I know, is popular, because people of all ages can do it. It's a little gentler on the bones. I've seen 70-year-old halmeoni doing it. Revisionist history also makes Taekkyon popular as it's seen as a traditional martial art. I'd call it more of a folk sport though. I'd liken it to modern boxing as a sport.
Muay Thai is getting really popular here. There are two gyms in my fiance's small town, so it's definitely getting out there. This is definitely male dominated. One female Korean champion teaches at an MMA/Taekwondo school in Itaewon though.
BJJ is creeping in slowly. There aren't many true black belts here. The only schools I know of, that are run by the same black belt, are in Yong-in (I think), and Apgujeong (definitely).
MMA is popular here, and gyms are popping up all over. They usually combine Muay Thai, western boxing, Taekwondo (to keep it linked to Korea), Judo, and some BJJ.
You can also take Korean traditional archery, which is kind of dying out. It's cool, and you shoot very long distances, like 100 meters.
If I can think of anything else, I'll let you know.
