Something as light as buffer switching. So I can quickly jump there and back to check, for example, the output of the last !-invoked command.
I know there is the :sh command, but then you have to exit it to get back to Vim, so it is a bit heavy for the purpose.
I am also aware of http://vim.wikia.com/wiki/Display_output_of_shell_commands_in_new_window but I am not a huge fan of splits.