Question

I want to add this code:

<link rel="stylesheet" type="text/css" href="css/global.css" />
<!--[if lt IE 8]>
    <link rel="stylesheet" type="text/css" href="css/iehacks.css" />
<![endif]-->

to all the files in a folder right before the </head> tag

Answer 1

I would record a macro to insert the text, then do a :bufdo to apply it to all open files.

Example:
Open all files in vim, then record a macro in register x with qx, go to the top of the file with gg, search for the end of the head-tag with /\/head, open a line before that with O (big O), then type or paste your text as usual, write your file with :w, end the recording with q.

After that, run :bufdo normal @x to apply it to all open buffers.