GConf
GNOME 2's settings database, a hierarchy of typed keys behind a daemon, which made programs react to changes instantly and made critics call it the Linux registry.
Havoc Pennington wrote it ahead of GNOME 2 (2002) as an answer to every program keeping its own config file in its own format. Keys were named like paths, /apps/metacity/general/focus_mode, carried a type and a schema with a default and a description, and were stored as XML under ~/.gconf. The daemon gconfd-2 sat in between: when one program wrote a value, everyone listening on that key heard about it at once, so a new theme took effect without a restart, the way XSETTINGS did for X programs. Administrators could place mandatory values in a system directory the user could not override.
The comparison with the Windows registry came immediately, and Pennington tried to head it off in advance, without success. The daemon speaking CORBA through the Bonobo stack did not help either. GNOME 3 replaced it in 2011 with GSettings, an API in GLib, and dconf, Ryan Lortie's backend with a single binary database read through mmap and written through a small service over D-Bus. The keys look almost the same; it was the middleman people objected to.