Geeky as hell, but I need it quite often, and what better place to put it but here: the list of 250+ HTML character entities.
Handy if you would want to write σ = ¾ • ∑ ƒ(χ²) – or your name like "πετερ"

UPDATE: now an even more complete overview can be found on Online Character map. Convert your own text to HTML-encoded with HTML encode.

A

á : [á] – Á : [Á] – â : [â] – Â : [Â] – ´ : [´] – æ : [æ] – Æ : [Æ] – à : [à] – À : [À] – ℵ : [ℵ] – α : [α] – Α : [Α] – & : [&] – ∧ : [∧] – ∠ : [∠] – å : [å] – Å : [Å] – ≈ : [≈] – ã : [ã] – Ã : [Ã] – ä : [ä] – Ä : [Ä]

B

„ : [„] – β : [β] – Β : [Β] – ¦ : [¦] – • : [•]

C

∩ : [∩] – ç : [ç] – Ç : [Ç] – ¸ : [¸] – ¢ : [¢] – χ : [χ] – Χ : [Χ] – ˆ : [ˆ] – ♣ : [♣] – ≅ : [≅] – © : [©] – ↵ : [↵] – ∪ : [∪] – ¤ : [¤]

D

† : [†] – ‡ : [‡] – ↓ : [↓] – ⇓ : [⇓] – ° : [°] – δ : [δ] – Δ : [Δ] – ♦ : [♦] – ÷ : [÷]

E

é : [é] – É : [É] – ê : [ê] – Ê : [Ê] – è : [è] – È : [È] – ∅ : [∅] –   : [ ] –   : [ ] – ε : [ε] – Ε : [Ε] – ≡ : [≡] – η : [η] – Η : [Η] – ð : [ð] – Ð : [Ð] – ë : [ë] – Ë : [Ë] – € : [€] – ∃ : [∃]

F

ƒ : [ƒ] – ∀ : [∀] – ½ : [½] – ¼ : [¼] – ¾ : [¾] – ⁄ : [⁄]

G

γ : [γ] – Γ : [Γ] – ≥ : [≥] – > : [>]

H

↔ : [↔] – ⇔ : [⇔] – ♥ : [♥] – … : […]

I

í : [í] – Í : [Í] – î : [î] – Î : [Î] – ¡ : [¡] – ì : [ì] – Ì : [Ì] – ℑ : [ℑ] – ∞ : [∞] – ∫ : [∫] – ι : [ι] – Ι : [Ι] – ¿ : [¿] – ∈ : [∈] – ï : [ï] – Ï : [Ï]

K

κ : [κ] – Κ : [Κ]

L

&lambda; : [λ] – &Lambda; : [Λ] – &lang; : [⟨] – &laquo; : [«] – &larr; : [←] – &lArr; : [⇐] – &lceil; : [⌈] – &ldquo; : [“] – &le; : [≤] – &lfloor; : [⌊] – &lowast; : [∗] – &loz; : [◊] – &lrm; : [‎] – &lsaquo; : [‹] – &lsquo; : [‘] – &lt; : [<]

M

&macr; : [¯] – &mdash; : [—] – &micro; : [µ] – &middot; : [·] – &minus; : [−] – &mu; : [μ] – &Mu; : [Μ]

N

&nabla; : [∇] – &nbsp; : [ ] – &ndash; : [–] – &ne; : [≠] – &ni; : [∋] – &not; : [¬] – &notin; : [∉] – &nsub; : [⊄] – &ntilde; : [ñ] – &Ntilde; : [Ñ] – &nu; : [ν] – &Nu; : [Ν]

O

&oacute; : [ó] – &Oacute; : [Ó] – &ocirc; : [ô] – &Ocirc; : [Ô] – &oelig; : [œ] – &OElig; : [Œ] – &ograve; : [ò] – &Ograve; : [Ò] – &oline; : [‾] – &omega; : [ω] – &Omega; : [Ω] – &omicron; : [ο] – &Omicron; : [Ο] – &oplus; : [⊕] – &or; : [∨] – &ordf; : [ª] – &ordm; : [º] – &oslash; : [ø] – &Oslash; : [Ø] – &otilde; : [õ] – &Otilde; : [Õ] – &otimes; : [⊗] – &ouml; : [ö] – &Ouml; : [Ö]

P

&para; : [¶] – &part; : [∂] – &permil; : [‰] – &perp; : [⊥] – &phi; : [φ] – &Phi; : [Φ] – &pi; : [π] – &Pi; : [Π] – &piv; : [ϖ] – &plusmn; : [±] – &pound; : [£] – &prime; : [′] – &Prime; : [″] – &prod; : [∏] – &prop; : [∝] – &psi; : [ψ] – &Psi; : [Ψ]

Q

&quot; : ["]

R

&radic; : [√] – &rang; : [⟩] – &raquo; : [»] – &rarr; : [→] – &rArr; : [⇒] – &rceil; : [⌉] – &rdquo; : [”] – &real; : [ℜ] – &reg; : [®] – &rfloor; : [⌋] – &rho; : [ρ] – &Rho; : [Ρ] – &rlm; : [‏] – &rsaquo; : [›] – &rsquo; : [’]

S

&sbquo; : [‚] – &scaron; : [š] – &Scaron; : [Š] – &sdot; : [⋅] – &sect; : [§] – &shy; : [­] – &sigma; : [σ] – &Sigma; : [Σ] – &sigmaf; : [ς] – &sim; : [∼] – &spades; : [♠] – &sub; : [⊂] – &sube; : [⊆] – &sum; : [∑] – &sup; : [⊃] – &sup1; : [¹] – &sup2; : [²] – &sup3; : [³] – &supe; : [⊇] – &szlig; : [ß]

T

&tau; : [τ] – &Tau; : [Τ] – &there4; : [∴] – &theta; : [θ] – &Theta; : [Θ] – &thetasym; : [ϑ] – &thinsp; : [ ] – &thorn; : [þ] – &THORN; : [Þ] – &tidle; : [&tidle;] – &times; : [×] – &trade; : [™]

U

&uacute; : [ú] – &Uacute; : [Ú] – &uarr; : [↑] – &uArr; : [⇑] – &ucirc; : [û] – &Ucirc; : [Û] – &ugrave; : [ù] – &Ugrave; : [Ù] – &uml; : [¨] – &upsih; : [ϒ] – &upsilon; : [υ] – &Upsilon; : [Υ] – &uuml; : [ü] – &Uuml; : [Ü]

W

&weierp; : [℘]

X

&xi; : [ξ] – &Xi; : [Ξ]

Y

&yacute; : [ý] – &Yacute; : [Ý] – &yen; : [¥] – &yuml; : [ÿ] – &Yuml; : [Ÿ]

Z

&zeta; : [ζ] – &Zeta; : [Ζ] – &zwj; : [‍] – &zwnj; : [‌]