Ranked in order of the 'value' to me.
View Manager
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CalibreSpy
Job Spy
Modify ePub
Count Pages
Drop Search Results
GetFileName
Find Duplicates
Extract ISBN
Reading List
*Quality Check
*Diaps Editing Toolbag
*EpubSplit
*Resize Cover
*User Category
* not convinced I need these
Added: I moved your post to here, plugins are regarded as part of the calibre furniture to help manage calibre libraries.
Related Tools are third party applications that integrate, or are used in conjunction, with calibre; servers, cloud, network, backup and others.
BR