안녕하세요! Koreanstudyjunkie입니다~ I'm in South Korea for the first time ever and it's been about 3 days so far, but there are a few apps I feel I would be lost without right now... in fact I got lost for like 2.5 hours... but let's not dwell on that.
Kakao Maps + Naver Maps
Coming to a foreign country, especially for the first time It's incredibly easy to get lost. Although Korea has some of the best public transportation around, It can still be confusing to travel using subways and/or buses. Naver maps is THE best app for getting around Korea. It can be a little bit of a learning curve when it comes to these apps, but they allow you to put in a starting point and an end destination. From here, you'll get directions on how to reach those places and also an approximate amount of time that it will take you. Which subway or bus stop to go to, when to transfer to where, and when it will show up.
Personally I prefer Naver Maps - at least when it comes to traveling via the subway. Naver maps unlike Kakao maps tells you precisely how many stops you should ride before getting off, which exit specifically you can take, and more. Kakao is almost the same thing, but it is a bit more difficult to use for me. Try both out and see which one you like more.
Kakao Taxi
Another essential app for traveling through Korea is Kakao Taxi. This has been a life saver for me as I chose an airbnb in Incheon - outside of Seoul. I stay kind of far from Seoul, so taking a taxi is sometimes my only option. Using this app you can request a general taxi to a certain place. By logging in you can access more feautires in the app. You can get a medium or large taxi - great for more than one person or someone with luggage that needs room.
In my experience, the drivers usually show up within a reasonable amount of time. Be aware if you cancel your taxi you may still be charged a fee. Taxis are more expensive than subways or buses, but it depends on if that is worth it to you or not. You can pay with credit/debit card, T-money card, or with cash (won) - but I will note that cash is becoming the less popular payment option.
Papago
If you don't speak Korean or know it that well then this is a great app. Whenever you need to quickly translate something this app will be a savior for you. Like most people, I consider this app to be much better than google translate but that said It's not perfect. Machine translation cannot entirely understand the context of every sentence, thus some sentences can be seen as awkward. That's your main issue with Papago - sounding unnatural - whereas with google translate you think you are saying one thing but you could be saying something entirely different - offensive even. In that case, I'd rather get my point across in an awkward way. For simple sentences the app is amazing and for more complex ones try to be careful how you phrase things - sometimes simplifying what you want to say can help.
Naver Dictionary & Translate
This one is more for my fellow Korean learners. By getting this app you'll also have access to Papago translation inside, but it comes with so much more. You can look uo words and find example sentences. There are daily Korean conversations and Idioms/Idiomatic expressions to learn. It is a lot better for those who are learning Korean, but anyone can use this. It has a bit of a learning curve to it, so don't give up too soon when trying it for the first time.
Kakao Talk (& kakao pay)
One of the most... no THE most popular messaging app in Korea is Kakao Talk. Pretty much everyone uses this app - even companies and businesses. You can keep in touch with friends and there are cute emoticons (not all are free). Certain apps may even ask for your Kakao ID - it's usually optional though.
Kakao pay is a payment form that is widely accept in Korea. You can pay for things through Kakao Talk with Kakao Pay - I have yet to do so. But I've heard that it is very convenient and you can send money anytime anywhere.
There are a few apps that I've seen recommended a lot, although I have not tried them all. Some delivery apps require a Korean cell phone number, so it didn't work for me. Shuttle didn't work, because I'm far from Seoul but it should work fine if you live in or close to Seoul - it doesn't require a Korean phone number.
HONORABLE MENTIONS:
Shuttle (delivery food app)
Coupang & Coupang Eats
Bike rental apps (ê¸±ê³ ìž‰)
This is my list so far! Like I said It's like day 3 for me right now, so I haven't got a ton of experience using all of the apps ever yet haha.