Ruby users, behold! Check this post. It is ruby autocomplete functionality for vim! As cuberick (the post author) points out, it needs ruby compiled with ruby support. It is distributed with vim and as part of the vim-ruby project according to the official rubycomplete.vim page.
If that is not enough you can even apply the completions with the TAB key by using the SuperTab vim script.
I am replicating for convenience (i.e. backup) the .vimrc chunk needed for all this magic to work:
"ruby autocmd FileType ruby,eruby set omnifunc=rubycomplete#Complete autocmd FileType ruby,eruby let g:rubycomplete_buffer_loading = 1 autocmd FileType ruby,eruby let g:rubycomplete_rails = 1 autocmd FileType ruby,eruby let g:rubycomplete_classes_in_global = 1 "improve autocomplete menu color highlight Pmenu ctermbg=238 gui=bold
As pointed out by cuberick, the last line fixes the default rubycomplete color scheme.