Are there any portable tools for directing sound output straight back in as microphone input?
EDIT I would prefer that it be software as apposed to hardware (a wire).
Virtual Audio Cable is a paid one I use and it works very well, it is the gold standard and it has been around since Win98 and it works all the way up to windows 7. It has a demo to try it out, but if you don't pay for it every so often it will say "Trial" over the output.
Also note (per the VAC page) you may already have the ability to do this with your stock windows drivers. Here is a tutorial on how to check to see if it is available for you on windows 7.