Lee Seung-gi as himself in The Greatest Love and The Producers. The former is one of my favourite k-drama cameos because of the way it plays on Seung-gi's 'perfect man/nice guy' image.

Conan O'Brien in One More Happy Ending

Choi Ji-woo as herself in Crash Landing on You

Park Bogum on the Producers.

There were a load of actors on Behind every star and Sh**ting stars but I don’t know how many were just themselves or if they were all playing actors.

Most recently, to me Jo Yeo Jeong's cameo was the best part of BEHIND EVERY STAR.

Chan Sung in Midnight Runners.

Junsu in Scent of a Woman.

Practically everybody in The Producers??

In Entourage (Episode 1), Seo Kang-Joon's movie star character chats with Ha Jung-woo, director Park Chan-wook, and Kim Tae-ri at a film festival.

Red Velvet performs for the soldiers in Descendants of the Sun (Episode 16) while Song Joong-ki and Jin Goo's characters fanboy and dance along.

Yoon Shi Yoon in "My absolutely boyfriend".

Park Sung Woong un "Eulachacha Waikiki" (LOL XD).

I feel like it's a fading trend - there used to be so many such instances a decade ago or so (esp during idol drama rage), but now writers are trying to be creative and invent new characters with fictional backstories even for cameos that last barely a few seconds. Maybe that's a good approach signifying an attempt to actually INCORPORATE star cameos in the plot and not just lure viewers in with random celebs popping up... maybe SK public is just THIS fed up with its stars being everywhere for no reason. Probably not tho)))

Junho did it in Uncontrollably Fond - the only scene I've seen from that show.

PD Na Young-seok. He cameoed as himself in Hospital Playlist as U-ju's potential father-in-law but with a different surname. It was really funny with Ik-joon telling him he very much resembled PD Na and he(PD Na) vehemently asserting he has no similarities with PD Na.
Also in Hospital Playlist, these two well established tennis players , National Team and Olympics Champions cameod as Nuclear Medicine doctors.

Jang Nara cameod herself in Shooting Stars. That was like a few months after her wedding.

Behind the Stars had actor cameos I very much enjoyed, with Cho Yeo-jung topping the list.

Hey! this should be so fun! I love seeing actors in cameos playing themselves, sometimes as a different character from a different drama and sometimes as oh, say, a hyped up version of themselves!

Kim Do-yeon in BE MELODRAMATIC, along with others of Weki Meki.

Kim Shin-young in ANOTHER OH HAE-YOUNG. Also maybe Lee Byung-joon.

Seo In Guk in She was pretty.
Choi Jin Hyuk in Entourage.

Behind Every Star had a lot of name dropping where actors went by their names.

Lastly, Celebrity had some guests like Lee Sang Yoon and David Lee Mcinnis at the opening. They were some youtuber/ influencers, dancers, models too.

In Was it Love?, SF9 did a cameo as a group.

I think it was in the The Best Hit where K-pop group Monsta X did a really brief cameo in one of the episodes!

Also I'll cheat slightly by mentioning the Park Bo-gum cameo in Itaewon Class - I think he was just the 'new chef' so they didn't introduce him as anyone but they also didn't not say he wasn't PBG soooo.....

Im Siwan in Thirty Nine.

Park Seo-joon in Record of Youth. I think he appeared as himself.

Does Jo In-sung’s cameo as CEO Ma’s car mirror reflection in the 500 Days of Summer dance sequence in Because This is My First Life count? Hee hee…

Start-Up features a cameo appearance by superstar pitcher Park Chan-Ho (known in the US as Chan Ho Park), who in 1994 became the first South Korean-born player in the history of Major League Baseball. I’m a longtime baseball fan, and he was the very first Kdrama cameo performer I was able to recognize!

One cute one is Go Ara in The Producers. She plays herself as the rival/pretend-BFF to IU's Cindy.