Nodewebkit and CEF are some of the main alternatives. And really, Electron is "just" something that does a lot of housekeeping for you, but at the end of the day you could do it yourself - you ship Chromium (or WebKit or ...) libraries, then create a native container window and then hand its drawing context off to the browser instance, shuttle events back and forth, etc.

CEF is kinda clunky, but also has bindings for just about any language.