What is this.....

... and where did it come from?

I've got the most interesting plant growing in my front yard. I didn't plant it, and it wasn't here the first few years that I lived here. It was probably about 3 years ago when this plant popped up for the first time. I love it, and I would buy a few more of them, if I knew what it was. I know it's a member of the lilly family-- probably a "cousin" of the Easter Lilly. It is a bushy plant that sends up great big, thick stalks. At the top of each stalk there are about 7-9 beautiful white flowers (that look exactl like Easter Lillies). The weight of the flowers always sends the stalk down to the ground. I've started cutting the stalks as soon as they bloom, and bringing the flowers into the house so I can enjoy them for a few days, rather than seeing them rot on the ground. This plant blooms for several months each year.

Does anyone have a clue what kind of plant this is? I'd love to know!