Viki, Dramafever, Crunchyroll, OnDemandKorea, Kocowa*, Tubi TV, certain official YouTube channels like KBS World... Netflix if you have it... These are the legal ones coming to mind.
You can also read this: https://kisskh.at/discussions/suggestions/27812-active-list-of-streamingdownload-sites?postid=672686#p672686
*I think Kocowa is mostly pay-only but they do have a free section, and you can check the Taste24Hr feature where they put new episodes of airing shows up for a while.