If you activate a Compose key on your keyboard:

Then you can insert Unicode characters without memorizing their hex values. Of course, you must memorize the Compose key sequences. Fortunately, they’re more-or-less mnemonic for the ones I occasionally use, which are hereby cherrypicked from that list.

Press-and-release the Compose key (right-Win), then type the characters as shown to get the symbol in quotes:

• o c “©” copyright # COPYRIGHT SIGN
• o o “°” degree # DEGREE SIGN
• o r “®” registered # REGISTERED SIGN
• t m “™” U2122 # TRADE MARK SIGN
• s m “℠” U2120 # SERVICE MARK
• . . “…” ellipsis # HORIZONTAL ELLIPSIS
• . – “·” periodcentered # MIDDLE DOT
• . = “•” enfilledcircbullet # BULLET
• + – “±” plusminus # PLUS-MINUS SIGN (∓ MINUS-PLUS is U2213)
• x x “×” multiply # MULTIPLICATION SIGN
• < < “«” guillemotleft # LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
• > > “»” guillemotright # RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
• c / “¢” cent # CENT SIGN
• – – . “–” U2013 # EN DASH
• – – – “—” U2014 # EM DASH
• < – “←” U2190 # LEFTWARDS ARROW
• – > “→” U2192 # RIGHTWARDS ARROW
• = > “⇒” U21D2 # RIGHTWARDS DOUBLE ARROW
• ? ! “‽” U203D # INTERROBANG
• p o o “💩” U1F4A9 # PILE OF POO
• m u “µ” mu # MICRO SIGN
• d i “⌀” U2300 # DIAMETER SIGN
• 1 4 “¼” onequarter # VULGAR FRACTION ONE QUARTER
• 1 2 “½” onehalf # VULGAR FRACTION ONE HALF
• 3 4 “¾” threequarters # VULGAR FRACTION THREE QUARTERS
• 1 1 0 “⅒” U2152 # VULGAR FRACTION ONE TENTH (and similar)
• ^ 1 “¹” onesuperior # SUPERSCRIPT ONE (also 0 2 3 + -…)
• _ 1 “₁” U2081 # SUBSCRIPT ONE (also 0 2 3 + -…)
• e ‘ “é” eacute # LATIN SMALL LETTER E WITH ACUTE
• e ` “è” egrave # LATIN SMALL LETTER E WITH GRAVE

Producing Greek letters requires a “dead_greek” key, so it’s easier to start with bare hex Unicode values at U0391 (Α) and U03b1 (α) and work upward until you find what you need:

• U03a9 Ω uppercase omega
• U03c9 ω lowercase omega
• U03c4 τ lowercase tau
• U03c0 π lowercase pi
• U0394 Δ uppercase delta
• U03F4 ϴ uppercase theta
• U03B8 θ lowercase theta
• U03D5 ϕ phi math symbol
• U03A6 Φ uppercase phi
• U03C6 φ lowercase phi

Odds and ends:

• U00a0   non-breaking space (invisible!)
• U2011 ‑ non-breaking hyphen
• U2245 ≅ approximately equal
• U2264 ≤ less-than or equal
• U2265 ≥ greater-than or equal
• U221A √ square root
• U221B ∛ cube root
• U221C ∜ fourth root (yeah, right)
• U221D ∝ proportional to
• U2300 ⌀ diameter

If you set the keyboard layout to US International With Dead Keys, maybe you (definitely not I) could remember all the dead keys.