seeks.vim (GitHub: sanpii / seeks.vim) by Sanpi is an interface to the Seeks.fr search engine that can display results using a window inside Vim. It's built using the webapi-vim project, which supports lots of interesting web-related technologies.
Gocode (License: MIT) by "nsf" is a daemon written with Go that provides context-sensitive autocompletion for various editors including Vim. It's designed to be fast, providing omni completion matching that would potentially perform quite slowly inside a typical editor:
Typical autocompletion time with warm cache is 30ms, which is barely noticeable.