Fairly often I want to find out whether there is an HTML entity for a given Unicode character, or given an HTML entity I want to look up its Unicode value.
For example, ∃ has Unicode value U+2203. I might want to look up whether there is an HTML entity for this. There is, and it’s ∃.
Another example is ○ (U+25CB). There is no HTML entity for this character.
I imagine a few other people need to do this occasionally, so I make a web page to make it easier.
The page is very similar to my page for converting back and forth between Unicode and LaTeX commands.
I’ve written a few other online tools. Here’s a full list.
Update: If you enter a single character, the page will look up the Unicode value of the input. For example, entering ∀ is the same as entering U+2200.