Other sites that have Xojo / Real Studio addons and plugins:

Bkeeney Software - Various of Xojo classes / addons and knowledge
Thomas Templemann's page - TT also has the excellent ZIP Archive classes
MacOSLib - Macoslib A Macintosh library to use advanced features which are not available in Xojo
MBS Plugins, a lot of plugins for REAL Studio.
VanHoek Software (REAL Studio plugins)
Valentina - The incredibly fast and powerful database engine.

Xojo / Real Studio developer related links:

Xojo Inc - the makers of REAL Studio
Ohanaware - App Wrapper and other tools that Xojo Mac developers should not be without.
Lua (The makers of the Lua interpreter)

Sites that have classes that extend Einhugur technology:

Ben Blake's Grid printing modules