The beaTunes API allows the implementation of plugins and beaTlets. For an introduction into plugin programming, please see the documentation on the main website. Key classes in this documentation are Plugin, BeaTunes, and AudioSong.
Packages 
Package Description
com.tagtraum.audiokern
Core audio classes for decoding, playing and analyzing audio data.
com.tagtraum.audiokern.audioprocessor
Classes that implement the SignalPullProcessor or SignalProcessor interface.
com.tagtraum.audiokern.beats
Code dealing with beat tracking, beatgrids and alike.
com.tagtraum.audiokern.danceability
Algorithms dealing with danceability.
com.tagtraum.audiokern.device
Audio devices ranging from default devices over mixers to AirPlay.
com.tagtraum.audiokern.key
Classes dealing with tonality, keys etc.
com.tagtraum.audiokern.lowlevel
Classes for calculating low level features.
com.tagtraum.audiokern.math
Support classes for mathematical operations.
com.tagtraum.audiokern.mood
Core classes for moods connected to music.
com.tagtraum.audiokern.player
Audio player implementations and utility classes.
com.tagtraum.audiokern.psychoacoustics
Classes having to do with how humans perceive audio.
com.tagtraum.audiokern.replaygain
Classes related to the Replay Gain Proposal by David Robinson.
com.tagtraum.audiokern.segments
Code dealing with segmentation and similarity.
com.tagtraum.audiokern.timesignature
Core audio classes for time signatures.
com.tagtraum.beatunes
Application for analyzing songs and sorting them based on how they match each other.
com.tagtraum.beatunes.action
Contains all significant actions.
com.tagtraum.beatunes.action.player
Actions relevant for the music player.
com.tagtraum.beatunes.action.standard
Standard actions like "Create Playlist".
com.tagtraum.beatunes.analysis
Provides classes to steer the execution of analysis tasks and several analysis tasks and their editors.
com.tagtraum.beatunes.analysisqueue
Provides an implementation for the analysis queue and its UI.
com.tagtraum.beatunes.charts
Amazon charts UI code.
com.tagtraum.beatunes.contextshop
Provides classes for the context shop.
com.tagtraum.beatunes.draganddrop
Drag and drop support code.
com.tagtraum.beatunes.filter
Provides classes for Swing JTable filtering and sorting.
com.tagtraum.beatunes.genre
Classes related to handling genres and genre ontologies.
com.tagtraum.beatunes.inspection
Allows the inspection of songs for issues.
com.tagtraum.beatunes.keyrenderer
Implementations for KeyTextRenderer.
com.tagtraum.beatunes.library
Classes that load and manage the library.
com.tagtraum.beatunes.library.filesystem
Implementation of a file system based music library.
com.tagtraum.beatunes.library.itunes
Implementation of an iTunes-based music library.
com.tagtraum.beatunes.library.plain
"Plain" library that does not have side-effects.
com.tagtraum.beatunes.library.playlistexporter
Playlist exporter implementations.
com.tagtraum.beatunes.library.recovery
Classes for recovering or deleting a corrupt H2 database.
com.tagtraum.beatunes.library.schemaevolution
Classes for evolving the internal database's schema.
com.tagtraum.beatunes.library.type
Special Hibernate types for color, mood etc.
com.tagtraum.beatunes.matchlist
Classes to support building matchlists.
com.tagtraum.beatunes.matchtable
Everything that has to do with matching.
com.tagtraum.beatunes.messages
Simple UI support for messages.
com.tagtraum.beatunes.onlinedb
Classes for accessing the online database.
com.tagtraum.beatunes.plaf
Provides classes for manipulating the pluggable look and feel (plaf) of the application.
com.tagtraum.beatunes.player
Components for the music player interface.
com.tagtraum.beatunes.playlisttree
Classes for the main source (tree) view, found on the left side of the main UI.
com.tagtraum.beatunes.plugin
Dealing with plugin related UI items.
com.tagtraum.beatunes.remotecontrol
Remote control classes.
com.tagtraum.beatunes.songinfo
Song info/edit pane.
com.tagtraum.beatunes.songtable
Classes for the main song table (SongTable) including FindBar.
com.tagtraum.beatunes.songtable.renderer
Table cell renderers.
com.tagtraum.beatunes.tag
Support classes for tags.
com.tagtraum.beatunes.transitionissue
Logic, that finds problems in song transitions.
com.tagtraum.beatunes.wikiinfo
Song context component that displays data from several sources, among them Wikipedia.
com.tagtraum.core
Core classes, unspecific to a particular application.
com.tagtraum.core.animation
Support classes for Swing based animations.
com.tagtraum.core.app
Base classes for building a plugin-based, UI-driven application.
com.tagtraum.core.cache
Caches.
com.tagtraum.core.image
Utility classes for applying special effects to images.
com.tagtraum.core.imageio
Utility classes for loading images.
com.tagtraum.core.json
JSON classes for libraries.
com.tagtraum.core.lang
Classes dealing with natural language.
com.tagtraum.core.metric
Support for collecting metrics (similar to logs).
com.tagtraum.core.ml
Basic machine learning support.
com.tagtraum.core.osx
Classes for macOS specific features.
com.tagtraum.core.rdf
com.tagtraum.core.social
Social services integration.
com.tagtraum.core.swing
Generic Swing components.
com.tagtraum.core.util
Utility classes.
com.tagtraum.core.win
Windows specific classes.
com.tagtraum.core.wrap
Annotation based wrapping.
com.tagtraum.genres
Genre ontologies.
com.tagtraum.genres.rdf
RDF-based genre ontologies.
com.tagtraum.h2migration
Migration from older versions of H2.
com.tagtraum.tunes
Platform independent Java API for iTunes.
com.tagtraum.ubermusic
Classes to lookup meta (über) information about music files.
com.tagtraum.ubermusic.acousticbrainz
Web service implementing the UberMusic interface to download high level data from the AcousticBrainz.
com.tagtraum.ubermusic.amazon
Offers access to Amazon and basic caching infrastructure.
com.tagtraum.ubermusic.bandcamp
Bandcamp.
com.tagtraum.ubermusic.beatport
Beatport implementation for the Chart interface using the API.
com.tagtraum.ubermusic.coverartarchive
Web service implementing the UberMusic interface to download artwork from the Cover Art Archive.
com.tagtraum.ubermusic.dbpedia
Access to DBpedia.
com.tagtraum.ubermusic.discogs
Discogs implementation for the ubermusic interfaces using the public API.
com.tagtraum.ubermusic.lastfm
Last.FM webservice.
com.tagtraum.ubermusic.luckylyrics
Implementation of LyricsProvider.
com.tagtraum.ubermusic.wikidata
UberMusic implementation based on Wikidata