Vim: Take the word under the cursor, append the current buffer's extension, and open it using find.

gistfile1.vim

nnoremap <silent> <C-o> :call FindFile()<CR>

function! FindFile()
   " Get the word under cursor.
   let cursorWord = expand("<cword>")
   " Get the current file name and keep only the extension.
   let currentFile = expand("%")
   let extPos = stridx(currentFile, ".")

   " Append an extension only if the current file has an extension.
   if extPos != -1
      let extension = strpart(currentFile, extPos)
   else
      let extension = ""
   endif

   " Construct the file name.
   let fileName = cursorWord.extension

   " Open the file in the current buffer.
   execute "find ".fileName
endfunction