Gallery of installed fonts

font-manager allows you to make collections of fonts for manual categorization, etc. It also allows some searching, by font "type" which includes things like script, decorative, text, etc. It is in Debian.

It has a compare feature that lets you compare multiple fonts with your choice of text. It also has a browse screen that shows all fonts at once (well, or at least as many as fit on your screen, it has a scrollbar).

Note that it creates a ~/.fonts.conf if you don't already have one. You'll have to clean this up for newer fontconfigs (jessie and later, I believe).

gnome-specimen shows multiple fonts, but lacks any filtering (other than by name). It is in Debian:

Opcion font viewer is an open source program for viewing fonts. It is a Java jar, so it can be run with the following command:

java -jar Opcion_v1.1.1.jar

Project's SourceForge page

However, AFAIK this is not available in the repositories.