Αντίο. (antío)
Goodbye. (more final; for longer separations)

Γεια σου. (geia sou)
Bye. (informal; to one person)

Γεια σας. (geia sas)
Goodbye. (formal or plural)

Τα λέμε. (ta léme)
See you. (very common)

Θα τα πούμε. (tha ta poúme)
See you. / We'll talk. (common)

Καλό βράδυ. (kaló vrády)
Have a good evening.

Καλό Σαββατοκύριακο. (kaló Savvatokýriako)
Have a good weekend.

Καλή συνέχεια. (kalí synécheia)
Have a good rest of your day. (literally: good continuation)

Καλή επιτυχία. (kalí epitychía)
Good luck. (especially for exams, interviews, projects)

Να προσέχεις. (na prosécheis)
Take care. (informal; to one person)

Να προσέχετε. (na proséchete)
Take care. (formal or plural)