What is Spacewar in my Steam library?

You didn't buy anything. Spacewar is a tiny test app Valve gives developers so they can try out Steam features before their own game has a store page. Some indie builds and mods borrow its app ID (so does pretty much every cracked game), and launching one of those makes Steam add Spacewar to your library and show you as playing it. Harmless by itself. You can remove it from the library, but it comes back the next time whatever is using it runs.