You can never remember the character entity you need right now; or more likely you just don't know it, eg. someone wanted to know the Delta symbol earlier today (Δ = Δ as it happens). So, mainly so I can find it again quickly next time...
- W3C: XHTML 1.0 - DTDs, the source, very detailed, very hard to read.
- Webmonkey | Reference: Special Characters, easy to read but not complete.
- WaSP : Learn : Reference : Character Entities in HTML & XHTML, exhaustive, easy to read.