This winter, I got a birdie ball putting green, and while I love it, I have to say I'm even more impressed by the birdie ball. The folks were cool enough to toss in a box of birdie balls, and man, they're cool. If you like to hack up your yard or the local park, you've gotta check them out.
They're like a whiffle ball, but way better. They fly like a real golf ball- same height, trajectory, curvature, but no distance. The ball just gets up there and falls out of the sky. I hit my 7 iron a shade under 160 with a Pro V, and I hit a birdie ball about 40 yards with my 7. The best part is they make a really cool sound if you hit them well.
I've also used them to warm up if there's no driving range. You can smack a few off the first tee while you're waiting for the group in front of you, or around the practice area without worrying about hurting someone. I highly recommend them.