There is such an awesome conversation going on at inbound.org about professional salutations on email. The version you see below is a concise version of the salutation examples discussed there (if you don't have the time to go through every comment).

Hi First Name

The United States, (if you are writing to them in English) Turkey, Brazil, Ukraine, Spain, Portugal, Ireland, Israel, India, Singapore. This is typically true if you are sending emails to executives in the tech, internet, media industries and to an audience that's mostly young people.

Dear First Name, Dear Mr./Ms. Last Name

Poland, Germany, Japan, (if sending a formal email) Netherlands, Brazil, India, Singapore

Oi, First Name or Ola, First Name

This is Brazil's equivalent of Hi, but do notice the 'comma' that comes between the name and the salutation.

Caro, Cara, Prezado, Prezada

This is Brazil's equivalent of Dear. Caro is for a man and Caro is for a woman. Prezado is for a man and so on.

Bonjour First Name, Bonjour Monsieur Last Name

The French like to be formal. There isn't any 'Hi' in business communication. It's Bonjour or Bonjour Monsieur.

Salut or Buna Ziua

The Romanians go with an informal 'Salut' or a formal 'Buna Ziua' (meaning, Good day!). In fact, in informal contexts, it's pretty ok to skip the salutation altogether.

Shalom

Try your bit of Hebrew when you communicate with the Israeli.

Some bonus tips beyond professional salutations

1. The Japanese like it to be formal, polite, trustworthy, and explain a lot before asking for something
2. The Russians like to send and receive long emails. They prefer that you are knowledgeable than friendly. They take their time to respond
3. The Dutch like you to be open and direct

How does your country respond to opening lines/salutations in an email? Are there any nuances worth noting?