Catznip is the development viewer for RLVa written by Kitty Barnett. This is an extended reimplementation of the RestrainedLove API created by Marine Kelley. RLVa has been in circulation since mid 2009 and is independent, sharing no code with RLV. (See RLVa / RLV Differences).
RLVa goes from Catznip into many other viewers making it especially useful for script developers as it represents the future of RLVa often months ahead of the mainstream.