Formal:
* Greetings
* Good morning/afternoon/evening
* How do you do?
* Pleased to meet you.
Informal:
* Hey
* Hi
* What's up?
* How are you doing?
* Yo
* Sup
* Howdy
* Good to see you
* Long time no see
Creative/Unique:
* Ahoy (nautical)
* Salutations
* Peace out (slang)
* Top of the morning to you (Irish)
* Shalom (Hebrew)
* Namaste (Hindu)
Specific situations:
* Good day (formal, especially in some parts of the world)
* Welcome (when someone is arriving)
* Nice to meet you (for introductions)
* I'm so glad you're here (welcoming someone)
* How have you been? (if you haven't seen the person in a while)
To be extra polite:
* It's nice to see you again.
* It's a pleasure to meet you.
Remember to consider the situation and your relationship with the person when choosing an alternative to "hello."