It's a Fortnight Lily, Dietes, formerly known as Morea, and sometimes called African Iris. Whew. To make matters worse, it's in the iris family, not the lily family. I'm not sure if it's Grandiflora or the other one. I just love them and I have three of them in my yard.
Renee

Here is a link that might be useful: Dietes Fortnight Lily

Renee, you are da bomb! Thank you for solving this mystery. I am a native plant enthusiast, and am at a complete loss when it comes to non-natives.
Katie

They are ubiquitous here in Southern California. But I'll take the undeserved praise anyway...

I grow a variegated form in my garden. They are tough, tough plants here. If they get shabby looking, cut down to the ground before new growth starts, which is February here. They come back looking fresh.. Also a good time to divide when they get too "thick".