These tools help you to check and see if your website is downloading in a compressed format or not.
Dean Edwards' JavaScript compressor tool, does what it says and appears to do it well. Copy and paste your script into the top and click the button. He provides a 21kb version of the Prototype library on the site for download. That's a savings of 30kb!
Given an URI, this tool will test to see if the page is compressed, or gzipped (gzip). If not, it will present a table with download times based on the actual page size and different compression ratios.