There are few metadata editors out there. This is because many developers are uncomfortable with the idea of people editing their metadata. It's easy to conceive of the deception arbitrary editing of metadata allows anyone to do, from stealing other people's photos to creating illusions about when and where a picture was taken. Please think carefully before editing metadata: are you doing it for purposes of deception? Do you want to contribute to the distrustful mood surrounding many digital editing communities?
Including correct metadata with your photos can be a powerful tool, both for yourself and others. Knowing the time when a picture was taken can help you duplicate lighting conditions, for instance, and the date can help you remember what day precisely you made that Everest summit. Going back and critically looking through the camera settings and how it affected the final outcome of a picture can help you improve—looking to see which exposures created the best waterfall photographs, or which f-stops worked best for a flower macro. On many websites, such as DeviantArt, the metadata from your photos is included with the displayed picture, allowing other photographers to learn from your work as well. Knowing the metadata can be a powerful tool for improving photography—so making sure that it's correct is of the utmost importance.
Before taking pictures with a camera, it's a good idea to make sure that your time and date are set correctly: this is probably the most common reason why people feel the need to change the photo metadata on a given picture. For other settings, it might be a good idea to make sure that the .exif data is correct by taking a few test photos at known settings and checking the .exif data to see if it is reading correctly.