While you can buy a simple hanging bird bath like this real cheap at most any garden decor store, you could also build one out of a number of different objects such as a plate like this. I've even seen them made out of hub caps. However, I wasn't very compelled to take any pictures of those. Still building one of these birdbaths out of a plate and twine is a simple landscaping project. Choose an object, drill or nail 3 or more equal spaced holes, connect string or chain, add water.
Once you've created and hung your birdbath, you'll need to keep it in water if it's going to attract any birds. Of course, you can do this daily by hand. If you have an automatic sprinkler system, you can place the birdbath in the spray to fill when it runs. You could also run a drip line to it from the sprinkler system so that the plate fills every time it runs.