Besides many of the tricks by designing the site (sprites instead of multiple images), what are some ways to reduce bandwidth using the tools on IIS7? I'm using a Pentium D processor so I'm hesitant to put any stress on it and haven't touched compression, but has anyone else had success with it?
I'm currently using an average of 5GB a day and on very busy days I reach 10GB (across ~10 sites). I use very few images in my designs and I don't host any media (video or mp3).
What are the best ways to find out what is using the bandwidth and how can I reduce it?
Thanks in advance!

AWStats (and probably other stats programs) can show you which pages and which types of files are consuming the most bandwidth.

I've been trying not to install any stats applications on my server because it's only a Pentium D with a small hard disk. I think I might not have a choice.

AWStats only processes once per day if you have it setup automatically, or when you manually do it. It's not continuously monitoring. Basically it just parses your IIS Logs and creates readable tables using the data it collects.

Have a read of this article for some ideas.

If its mostly text your webserver is serving then compression should yield considerable bandwidth savings even on the lowest compression setting. I would not use any of the higher settings as the resulting delays due to the extra cpu overhead mitigate the small increase in compression.