"Hey there" is one casual and friendly way to say "hello".

Here are a few situations in which you can say "Hey there" to greet someone:

• You're at a concert. You see someone that you work with, but you can't remember her name. You say "Hey there" when you walk by her.
• You're fishing on a small lake. A family rows by you in a row boat. You say "Hey there" and smile at them.
• You're friendly with one of the people who works at the local market. When you see him working there, you say "Hey there."