Open related file in vim (a la Sublime Text 2's Alt-O)

gistfile1.vim

" Opens a file in [path/to/current_file.*] that isn't already open.
function! RelatedFiles()
  return filter(split(globpath(expand('%:h'), expand('%:t:r').'.*')), '!bufloaded(v:val)')
endfunction
function! OpenRelatedFile()
  let files = RelatedFiles()
  if len(files) > 0
    " always open .h files on the left.
    if match(files[0], "\.h$") >= 0
      exec 'lefta vsplit '.files[0]
    else
      exec 'rightb vsplit '.files[0]
    endif
  endif
endfunction
nmap ^[o :call OpenRelatedFile()<CR>
nmap <M-o> :call OpenRelatedFile()<CR>
nmap ø :call OpenRelatedFile()<CR> " OSX is adorable
" Alt keys can be fiddly; you might need to replace <M-o> above with ^[o (literal ^[, type Ctrl-V Alt-O)