Straight from the Yahoo Developer Network: YSlow analyzes any web page and generates a grade for each rule and an overall grade. If a page can be improved, YSlow lists the specific changes to be made. Highly recommended tool :)
Being one of the zillion programmers jumping into rough waters of XML, XSLT and AJAX (combined with ASP back-end) I had my own share of "reinventing the wheel" experience. Hopefully by documenting some of these reinventions, I'll save you a few minutes of hair-pulling.