Yzis is a vi/vim engine that is easy to integrate in any graphical application. The goal is to make it easy for people developing an application that requires an editor to integrate yzis. Currently, there is a Qt, ncurses and KDE kpart yzis frontend.
Sonnet is a plugin-based spell checking library for Qt-based applications. It supports several different plugins, including HSpell, Enchant, ASpell and HUNSPELL.
It also supports automated language detection, based on a combination of different algorithms.
JuK is an audio jukebox application, supporting collections of MP3, Ogg Vorbis, and FLAC audio files. It allows you to edit the "tags" of your audio files, and manage your collection and playlists. It's main focus, in fact, is on music management.