* 뭐 해? (mwo hae?): This literally translates to "What are you doing?" It's a very common and informal way to greet someone casually, similar to "What's up?" Use this with close friends and family.
* 뭐 하고 있어? (mwo hago isseo?): This is a slightly more polite and formal version of "What are you doing?". Still informal, but a bit more respectful than just 뭐 해?.
* 잘 지내? (jal jinae?): This translates to "Are you doing well?" or "How are you?". This is a more polite and general greeting, suitable for friends and acquaintances.
* 오랜만이야! (oraenmania!): This means "Long time no see!" and is a suitable greeting if you haven't seen the person in a while.
Which one you use depends on your relationship with the person and the situation. For most casual situations with close friends, 뭐 해? (mwo hae?) is the closest equivalent to "What's up?".