Inspired by the title Killed by Google 
, I will mention some of my favorite software or ideas but no longer active or discontinued
Self has some of the coolest ideas for programming specifically objects. It is conceived long long ago wikipedia . There is a project available to tryout for Mac, it is usable demo to some extent github . It is a piece of history at this point.
Jef Raskin's The Humane Interface or Archy . This project has very interesting ideas for search using what is called as Leaping. There is quasimode Command functionality to allow one to hold a key like capslock and type to do many commands like running a code, opening a browser, starting a tutorial, and much more. The follow up by Jef's son Aza who created Enso my favorite launcher utility for Windows, has this quasimode command functionality in pretty interface also no longer active.
Xiki , ambitious project much like above Archy, to create a notepad or scratchpad like interface that takes plain text as input for both content and commands. It has very useable demo if you get your ruby environment to work.
Notion is a window manager for Linux. Notion has cool concepts, there are various tiling managers available but what they do is automatic layouts, in Notion you have full control over the layout, you can create your GUI frames, and any app you launch will load into the frame, it is completely customizable, has built-in launcher / menu system. It is not kept updated regularly, you can still manage to make it work.