Some Unicode characters that are occasionally handy:
U+0276D MEDIUM RIGHT-POINTING ANGLE BRACKET ORNAMENT
Handy as an ornamental separator. Used by
Triple back quotes.
<pre> tags for formatting things like Markdown or WikiText source code.
U+A937 REJANG LETTER BA
Useful for adding a forward slash to a Tiddler title. Might not be available in all fonts — it is best to avoid using slashes in titles altogether.
U+200B ZERO WIDTH SPACE
Useful for preventing the Markdown auto-dingus from converting things like (c) into ©.
U+FF20 FULL WIDTH COMMERCIAL AT
Useful for email addresses that would otherwise be converted into hyperlinks which can cause problems with the very fragile Markdown parser.
U+263A WHITE SMILING FACE
U+2639 WHITE FROWNING FACE
See: https://www.compart.com/en/unicode/block/U+FF00 for half/full width versions of common characters that might be useful when auto-markup features are getting in the way.