For multi line version you can do this after selecting the text:
:'<,'>:w !command<CR>
See the official Vim docs at :help :w_c
.
You can map it to simple Visual mode shortcut like this:
xnoremap <leader>c <esc>:'<,'>:w !command<CR>
Hit <leader key>+c in visual mode to send the selected text to a stdin of the command. stdout of the command will be printed below vim’s statusbar.
Real world example with CoffeeScript:
https://github.com/epeli/vimconfig/commit/4047839c4e1c294ec7e15682f68563a0dbf0ee6d