The Lightroom catalog is a SQLite database, and it is possible to write programs that can access it that can cooperate with Lightroom. That said, most developers that I have seen choose to do this in a supported way through the plugin route and the API Adobe provides. There are plugins which can modify metadata, for example:

Lightroom Transporter

I suspect however that is not quite what you want; I suspect you want something for manual individual image updates, fast.

The best I have seen is a pre-import tool. I use Photo Mechanic from Camerabits, which is probably the best known ingestion/browsing/metadata tool going. It works best to ingest, cull, crop, and describe photos, then you can drag and drop them directly into Lightroom which invokes the Import and carries over all the metadata added.

Photo Mechanic is all about speed, it is widely used by photo journalists on deadline, to get images in fast and tag fully, then push out to whoever needs it. I found it cuts my sports shoot post processing by at least 30% and maybe 50%, because I then use Lightroom only for what Lightroom does well - final develop, and publishing.

In theory you can use it after the fact and exchange metadata through the XMP files both use, e.g. import to lightroom, write metadata to XMP, then browse with Photo Mechanic which updates XMP if you change metadata, then read the metadata in Lightroom. But I would expect this could lead to getting confused easily, and overwriting changes from one in the other. I think it is better to use it up front where the leverage is (culling), then use Lightroom from then on. After all metadata in Lightroom is fast to update in the Library module.

I am not aware of any programs that can actually edit the image that is cross compatible with lightroom (i.e. go back and forth between non-destructive editing in lightroom and in program X); I suppose one could be reverse engineered but I have never seen any attempts at that.