"Hi there" is more American. It sounds more informal. I think it sounds a bit cheezy so I never use it.
In England you don't hear it a lot - mainly just from people who try to sell you something: "Hi there, can you spare a few minutes to talk about supporting Amnesty International...." and so on.

I think people say it more over the phone or with strangers, than face to face with somebody they know.

If you want to sound informal with friends, I agree with rockzmom that "Hey!" sounds nice. It's got an American feel to it but in a nice way. I use that sometimes.

Alex, I would recommend you not to use "Hi there". I know some non-native English speakers (Indians) who use it and it sounds silly together with their foreign accents.

Just say Hi, Hello or Hey - it is more universal.

Hello is slightly more formal than "Hi".

I often start work emails with "Hello" instead of "Hi" when I want so sound just a tiny bit more formal.