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

λ : [λ] – Λ : [Λ] – ⟨ : [⟨] – « : [«] – ← : [←] – ⇐ : [⇐] – ⌈ : [⌈] – “ : [“] – ≤ : [≤] – ⌊ : [⌊] – ∗ : [∗] – ◊ : [◊] – ‎ : [‎] – ‹ : [‹] – ‘ : [‘] – < : [<]

M

¯ : [¯] – — : [—] – µ : [µ] – · : [·] – − : [−] – μ : [μ] – Μ : [Μ]

N

∇ : [∇] –   : [ ] – – : [–] – ≠ : [≠] – ∋ : [∋] – ¬ : [¬] – ∉ : [∉] – ⊄ : [⊄] – ñ : [ñ] – Ñ : [Ñ] – ν : [ν] – Ν : [Ν]

O

ó : [ó] – Ó : [Ó] – ô : [ô] – Ô : [Ô] – œ : [œ] – Œ : [Œ] – ò : [ò] – Ò : [Ò] – ‾ : [‾] – ω : [ω] – Ω : [Ω] – ο : [ο] – Ο : [Ο] – ⊕ : [⊕] – ∨ : [∨] – ª : [ª] – º : [º] – ø : [ø] – Ø : [Ø] – õ : [õ] – Õ : [Õ] – ⊗ : [⊗] – ö : [ö] – Ö : [Ö]

P

¶ : [¶] – ∂ : [∂] – ‰ : [‰] – ⊥ : [⊥] – φ : [φ] – Φ : [Φ] – π : [π] – Π : [Π] – ϖ : [ϖ] – ± : [±] – £ : [£] – ′ : [′] – ″ : [″] – ∏ : [∏] – ∝ : [∝] – ψ : [ψ] – Ψ : [Ψ]

Q

" : ["]

R

√ : [√] – ⟩ : [⟩] – » : [»] – → : [→] – ⇒ : [⇒] – ⌉ : [⌉] – ” : [”] – ℜ : [ℜ] – ® : [®] – ⌋ : [⌋] – ρ : [ρ] – Ρ : [Ρ] – ‏ : [‏] – › : [›] – ’ : [’]

S

‚ : [‚] – š : [š] – Š : [Š] – ⋅ : [⋅] – § : [§] – ­ : [­] – σ : [σ] – Σ : [Σ] – ς : [ς] – ∼ : [∼] – ♠ : [♠] – ⊂ : [⊂] – ⊆ : [⊆] – ∑ : [∑] – ⊃ : [⊃] – ¹ : [¹] – ² : [²] – ³ : [³] – ⊇ : [⊇] – ß : [ß]

T

τ : [τ] – Τ : [Τ] – ∴ : [∴] – θ : [θ] – Θ : [Θ] – ϑ : [ϑ] –   : [ ] – þ : [þ] – Þ : [Þ] – &tidle; : [&tidle;] – × : [×] – ™ : [™]

U

ú : [ú] – Ú : [Ú] – ↑ : [↑] – ⇑ : [⇑] – û : [û] – Û : [Û] – ù : [ù] – Ù : [Ù] – ¨ : [¨] – ϒ : [ϒ] – υ : [υ] – Υ : [Υ] – ü : [ü] – Ü : [Ü]

W

℘ : [℘]

X

ξ : [ξ] – Ξ : [Ξ]

Y

ý : [ý] – Ý : [Ý] – ¥ : [¥] – ÿ : [ÿ] – Ÿ : [Ÿ]

Z

ζ : [ζ] – Ζ : [Ζ] – ‍ : [‍] – ‌ : [‌]