"Dear <name>" is the standard. "Dear Mr Peachey," is good for formality, but if you used my first name, then "Dear David," sounds more friendly. You can still use this if you haven't met me before. As Jura suggested, "Hi..." is very appropriate between friends, and when being friendly in general. (Hi Bobji! Hi Jura!) Sometimes you'll have to write a letter to someone you don't know at all, or don't like! You can use "To...", or just address them formally, such as "Mr Peachey," and sign off with a curt "regards". If you don't have an actual person to address, "To whom it may concern," is OK to use.

As Bobil says, it depends on whether the letter is formal or informal. If I'm writing to a friend, I just write Hi or Hi there.

I would say it depends on who the letter is to. If you are writing the letter to someone in your family or you know very well, you would probably say "Dear....". But "Dear..." is also used in business letters, as it is polite. If you are writing a letter to a friend, then you can say whatever you like. this page has a good list of the different ways to start letters and when to use the examples http://answers.google.com/answers/threadview/id/369194.html