Informal:
* Kumpel: This is the most common and versatile term, similar to "buddy" in English. It's appropriate for close friends and people you know well.
* Kolega: This literally translates to "colleague" but can also be used informally for friends, especially male friends.
* Koleżanka: The feminine form of "kolega", used for female friends.
* Ziom: This is a slang term for "friend" or "mate", similar to "dude" in English. It's typically used among young people and friends.
Formal:
* Przyjaciel: This means "friend" and is a more formal term, suitable for close friends or acquaintances you want to address respectfully.
* Towarzysz: This literally translates to "companion" and can be used for someone you're traveling or working with, or someone you share a common goal with.
Other options:
* Przyjacielu: This is the vocative form of "przyjaciel", meaning "friend", and is used when addressing a close friend.
* Kolego: This is the vocative form of "kolega", meaning "colleague", and is used when addressing a friend or colleague.
The best choice depends on the specific situation and your relationship with the person.