There have been a couple of additions to the official list of plugins in recent weeks. I thought I should make them more known. Maybe you find something you want to add to your tagging workflow.
Post Tagging Actions
This plugin by @twodoorcoupe allows you to specify external commands to run on albums or tracks. See the detailed documentation for instructions on how to use the plugin.
Add to Collection
This plugin by @dvitz allows you to add any saved release to one of your user release collections.
The plugin adds a settings page under the “Plugins” section under “Options…” from Picard’s main menu that lets you choose the collection you want to add the releases to.
We all agreed that this functionality should become part of Picard eventually 
Enhanced Titles
Another plugin by @twodoorcoupe. This one sets the albumsort and titlesort tags. For each track or album it first checks if there are any sort names already available in the aliases, otherwise it swaps the title’s prefix.
The plugin also provides language aware scripting functions $swapprefix_lang, $delprefix_lang and $title_lang. They work similar to the existing functions without the _lang suffix, but take an additional language parameter. E.g. $swapprefix_lang(%artist%,fra) will swap French articles at the start of the artist name to the end. Currently supported are English (eng), Spanish (spa), Italian (ita), French (fra), German (deu), and Portuguese (port).
Tagger script functions to detect if a file is lossless or lossy
This plugin by myself might need a catchier name. But what it does is providing two scripting functions $is_lossless() and $is_lossy(). Both are supposed to be used in file naming scripts and indicate if the tagged file is considered lossless or lossy. They are supposed to be used together with a condition like $if. E.g. if you want to put your files into separate folders “lossless” and “lossy” this snippet can do this:
$if($is_lossless(),lossless,lossy))/
Please note that the plugin does not actually inspect the file’s codecs in detail. It mostly looks at file extension and uses some bitrate heuristics for MP4 files. But it works well enough for most use cases.
I had first published this plugin some years ago here on the forums, see this discussion for some background.
Script Logger
This is actually already from 2023, but has seen little mention. This straight forward plugin by @rdswift adds a scripting function $logline(text[,level]) which can be used to write log messages to Picard’s error/debug log. E.g. $logline(artist name is %artist%,D) will log a debug message. Supported log levels are E (Error), W (Warning), I (Info) and D (Debug).