Did the mother abandon it?
This can often be answered by watching the baby animal for a while. With birds, 30 minutes should be ample time to see one or both of the parents deliver food to it. The parents will leave the fledgling in one place while they busily collect food to bring back. Even if you don't see the parents within 30 minutes, the general rule of thumb for birds is if the chick is fully feathered and out of its nest, it's more than likely doing just fine.
When it comes to baby rabbits (called "kittens" believe it or not!), the young achieve independence at a very young age, just 4-5 weeks. A rabbit small enough to fit in the palm of your hand is already old enough to fend for itself. If you find a nest of rabbits unattended, don't be alarmed. Just like with birds, the mother will leave her young for sometimes hours at a time in between nursings. As long as the young appear healthy, you can let them be.
Should I take the animal to a rehabber?
If the animal that you find does not seem to be doing well on its own as described above, I recommend that you first call a wildlife rehabilitation center such as the Schuylkill Wildlife Rehabilitation Center in Philadelphia before moving the animal. These professionals will ask questions to gauge the status of the animal and will be able to then make the proper call. Moving an animal unnecessarily may do more harm than good.
Should I try to help the animal myself?
This is not something that I would advise, even to someone who has a strong knowledge of wildlife. First of all, keeping a wild animal in your home, even for a short period of time, is against the law. Wildlife rehabilitators have licenses, and have undergone extensive training and testing as a requirement of their permit. Taking care of a wild animal, especially a baby, requires round-the-clock attention. As the saying goes, this work is best left to the professionals.
This spring, enjoy watching all of the wildlife babies around you and appreciate the tireless efforts of their dedicated parents. And if you happen to meet a wildlife rehabber this spring, thank them for their tireless efforts as well!