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.).
QuickFunctor is a C++ functor library with many improvements over the standard-provided implementation, as well as over some competing implementations in Boost, TR1, or Loki. Its strong points are arithmetic/boolean expressions with functors, a pretty complete set of operations to create new functors from existing ones, simplicity of types used, the ability to deal with an arbitrary number of parameters for functors, the ability to create a functor out of pretty much anything, and more.
ZebraTuner is a guitar tuner and generic instrument tuner. It compares frequencies detected in the signal coming from the microphone to standard or user-defined notes, displaying the results in both graphical and textual form. The number of notes tested and their frequency is configurable, so it can be used for other instruments and other tunings besides those based on equal temperament (e.g. just intonation-based tunings).