All writers I know love to eavesdrop. I tell kids when they ask What Does it Take to Be A Writer? that they must listen and remember.
Or in my case, listen and write it down.
The best place for eavesdropping is one where you'll blend in. A train ride, for example.
I like to name characters seen on the train and imagine their stories.
Here's an exchange between a boy in a Yankee cap and another kid, possibly his older sister.
I don't think I'll be using this. Go ahead and steal it if you'd like.
BOY (as we pulled into the South Orange stop on the NJ Transit Mid-Town Direct to NYC):
"This looks just like New York!"
GIRL: You've never even been to New York.
BOY: "Well, it looks just like what I've seen on Cash Cab."
What's the best eavesdropping conversation you've heard?
Or are you really going to tell me you never eavesdrop...
Check this site for even more confessions of writers eavesdropping.