A nice HTML & XHTML Character Entities reference is available at cookwood.com. It’s a pretty good go-to reference for web developers, I have to say. The site is the official site for HTML Visual Quickstart Guide and offers many other HTML related references.
As nice as this reference is I’ve been thinking about a better way to present HTML and a XHTML character entities in a chart that is inspired by the periodic table of elements. I’m actually moving forward with this project so look for an announcement on Digital Media Minute soon when I get it finished; hopefully people will find it has some general usage.