Calibre is an ebook collection manager with support for many services, including Amazon, Android or Barnes & Noble. Calibre makes it easy to organize ebooks, as well as search various services for new ebooks. The latest version is 1.33 and brings several improvements, like a tool to check the spelling and a number of bug fixes.
Clementine was started as a replacement for Amarok 1.4 after the transition to Amarok 2.x, which changed the look and behavior of the classic player. Clementine follows the exact same interface of Amarok 1.4.x series, getting new features with each release, currently being on par with the older, so much beloved player. Clementine is written in Qt and bundles many powerful features, a collection manager, integration to many services, song information and lyrics, equalizer and much, much more.
Audacious is an advanced, feature-complete music player for GNOME using GTK 3, that bundles a significant number of options, a clean interface, support for real-time audio effects, visualization, equalizer, lyrics and other plugins, themes (including Winamp Classic style with installable skins), powerful configuration options and multiple playlists organized in tabs.
Picking up one entry for each category was no easy task because applications are divided between GTK and Qt, and there are programs that run in the console and do their job perfectly and are more lightweight than a GUI application. As such, I decided to leave terminal applications out of this review in the first place, but I provided a link below to an article which covers those. Applications listed below are built using various toolkits, not just a single one like GTK, however they are among the most lightweight graphical tools from their category.