DeaDBeeF is an audio player for GNU/Linux and other UNIX-like systems. It is written in C with some plugins in C++. It has minimal dependencies, a native GTK2 GUI, cuesheet support, support for MP3, Ogg, FLAC, and APE, chiptune formats with subtunes, song-length databases, and more. It is very fast and lightweight, and extensible using plugins (DSP, GUI, output, input, etc.). The GUI looks similar to Foobar2000.
Guayadeque is a music player that aims to be easy to use and fast even for huge music collections. It allows you to automatically or manually download covers, easily organize the music library, edit the ratings of the tracks, add labels to the artists, albums, and tracks, listen to shoutcast radios, download lyrics, show artist information based on last.fm content, and more. It includes a simple, built-in tag editor with picture support.
MP3 Diags finds problems in MP3 files and helps the user fix many of them. It looks at both the audio part (VBR info, quality, normalization) and the tags containing track information (ID3). It has a tag editor, which can download album information (including cover art) from MusicBrainz and Discogs, as well as paste data from the clipboard. Track information can also be extracted from a file's name. Another component is the file renamer, which can rename files based on the fields in their ID3V2 tag (artist, track number, album, genre, etc.).
SvOlli's Little Audio Related Thingies, or SLART for short, is a compilation of rather simple programs that are related to audio files: it consists of a player (Partyman), a CD ripper, a tag editor, and some other tools. These programs can communicate with each other via a custom IPC bus, so a newly ripped track can be automatically queued in the player, for example.
irrKlang is a high level 2D and 3D cross platform sound engine and audio library that plays WAV, MP3, OGG, MOD, XM, IT, S3M, and other file formats. It can be used by C++ and all .NET languages (C#, Visual Basic.NET, etc.). It has all of the features common to low-level audio libraries as well as lots of useful features like a sophisticated streaming engine, extendable audio reading, single and multi-threading modes, 3D audio emulation for low-end hardware, a plugin system, multiple rolloff models, sound effects, and more. All of these can be accessed via an extremely simple API using an object oriented design.
K-Yamo is an MP3 and Ogg Vorbis tagger with ID3v2 support. It can store ID3v1, ID3v2.3, or ID3v2.4 tags. It also stores pictures in the tags. K-Yamo supports multi-file editing. It can guess tags out of filenames, and fetches the missing parts from musicbrainz. It also cleans up your files into a structured filesystem tree which makes use of the tags in the files. Finally, it is a CD ripper with FreeDB support. A MySQL database can optionally be used for tagging.
LibQtPod provides access to the contents of an Apple iPod. The code was formerly part of the kio ipodslave, but is now a separate Qt-based library. The library features reading and writing of the iTunesDB music database and provides access to hardware specific information like disc space statistics.