I usually just say, "Hello?" What now truly bothers me is that if it's a telemarketer there will be a significant pause, then "Hello?" from the other person. At that point I know they weren't listening when I answered, so they get nothing from me. Usually, though, I just let the answering machine get it; then if it's someone I want to talk to I'll pick up and let them know I'm home. (The answering machine message is either the computer-generated voice: "Leave a message, please" or my personal recording: "To speak or not to speak, that is the question. Whether it is nobler in the mind to leave a message after the beep or to take up arms against a sea of answering machines and, by opposing, end them. To dial, to speak no more. Thus do answering machines make cowards of us all. Leave a message after the beep . . . or don't.")
When I was a kid, I answered either with "Hello, (last name) residence" or just "Hello". My mother had a friend who always started with "Who is this?" Her voice was distinctive, and I always knew who it was, but one day I had had enough and replied, "Who wants to know?" She never did it to me again.
When I was studying Spanish in high school, we learned that a standard greeting in Mexico was, "Pues, diga!" (Well, talk - similar to the Italian, it's a way to let the person know you're ready to listen.) I would guess that it's pretty much changed to simply "Hola" in most places.