We may say "καλέ" when we address someone in Greek, similar to "βρε".
Τι κάνεις καλέ, είσαι καλά; (= Hey, what are you doing, are you ok?)
Τι κάνετε εκεί, καλέ; (= Hey, what are you doing over there?)
Kαλέ is actually more polite than βρε, because it comes from the adjective καλός, καλή, καλό (= good, nice), while βρε comes from the ancient adjective μωρός, μωρή, μωρό (= stupid).
Kαλέ is the vocative case of the masculine adjective καλός while βρε comes from the vocative case of the adjective μωρός.
For more information about "βρε" please see http://learn-greek-online.com/ask-greek/27/what-does-%CE%B2%CF%81%CE%B5-mean?
Καλέ can be used when we address a man, a woman or many people. It is an informal and friendly way to address someone and we never use it in formal occasions. It is considered a little feminine expression, that means you will hear more women than men say it.