Usually just heavy steel plate hanging from chains. We have all different sizes, so they make differnt 'dings', and we like to hit them in certain orders a pretend it's a song composition. Someone else mentioned hockey pucks, I have dengled those for shooting as well, a quick coat of a flourescent colour in a spray bomb makes them good to shoot, but it's not long before chunks of vulcanized rubber are all over the place.
In the winter, we'll just set up chunks of ice from the lake, or well-packed snowballs. Free, unlimited, and they smack apart dramatically when hit too. Lots of labour involved, but you can build a snowman and have some fun with that. We put together a big one once (over 5'), soaked it down and let it freeze solid overnight, and then shot at it from REALLY far (did not measure, but I would guess between 500 - 600). That was fun, but a lot of work to get set up.