CSF: ExifTool by Phil Harvey
Apologies if this tool was already mentioned.
Possibly the definitive meta data editor Exiftool by Phil Harvey
Certainly the best starting point for understanding file meta data (exif and all).
Command line interface, so probably not for the casual user.
For the longest time I edited w/ a combo of A Better Finder Rename, A Better Finder Attributes, GPS PhotoLinker, and SetEXIFData. The last two being GUI front ends to Exif Tool. Recently I started bringing my old home videos (Sony DV) into Photos. I wanted to tag the .mov files with geo-tags and other meta info. To get what I wanted (tags read on import to Photos) I needed to use exiftool.
So if you're working with photos the easier to use tools are great. If you're up against the edges of those capabilities (such as tagging mov files such that Photos will read it) you may want to look at Exiftool.
And if you want to geek out over meta data formats and standards definitely have a look at Phil's website.