I also do some custom development and have practical experience of a wide range of tools (I used to be a financial IT consultant specifying and implementing business systems) :
- XML, XMP
- Excel VBA developer
- SQL Server, some Oracle
Small ones can be done on an Amazon wish list basis.
Search Replace Transfer is for searching and replacing metadata like a word processor.
ListView is a plug-in for Lightroom that displays images in a list style just like in most other DAM (digital asset management) programs, and exports metadata directly to Excel or a browser.
Syncomatic synchronises Library and Develop settings between pairs of files with matching names or within stacks
Open Directly opens the original images directly in another program. The other program may be another raw converter, or any program the user chooses.
Capture Time to Exif is an in-Lightroom interface for Exiftool, the command-line application for reading, writing and editing meta information in raw files.
Locktastic is a simple plug-in that marks read-only or "locked" files with the red label.
FolderSize is a free plug-in that loops through all the selected images and analyses them by folder. It counts them and shows how much disc space they use.
Big Note is a very simple plug-in that adds a big note field to the Metadata panel. You can store any text in the field, and it can also be searched by smart collections.