Useful unicode characters I wish my keyboard had

gistfile1.txt

Greek:
  Α Β Γ Δ Ε Ζ Η Θ Ι Κ Λ Μ Ν Ξ Ο Π Ρ Σ Τ Υ Φ Χ Ψ Ω 
  α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ σ τ υ φ χ ψ ω
  
Superscript:
  x⁰ x¹ x² x³ x⁴ x⁵ x⁶ x⁷ x⁸ x⁹ x⁺ x⁻ x⁼ xⁱ xⁿ
  
Subscript:
  x₀ x₁ x₂ x₃ x₄ x₅ x₆ x₇ x₈ x₉ x₊ x₋ x₌
  xₐ xₑ xₒ xₓ xₔ

Math symbols:
  ∑ ∏ √ ∞ ±

Logic:
  ¬ ∧ ∨ ∃ ∀

Relations:
  ≠ ≤ ≥ < > ≡ ≢ ∼ ≅ ≈

Binary operations: 
  − × ÷ ⁄ ⊥ ⊕ ⊗ ⋅ ∗ ⨯

Punctuation:
  ′ ″ ‾ ° ⋅ · … – —

Arrows:
  ← ↑ → ↓ ↔ ↕ ↖ ↗ ↘ ↙ ↤ ↥ ↦ ↧ ↰ ↱ ↲ ↳ ↴ ↵ ↺ ↻
  ⇐ ⇑ ⇒ ⇓ ⇔ ⇕ ⇖ ⇗ ⇘ ⇙
  ⇦ ⇧ ⇨ ⇩ ⇪ ⇫ ⇬ ⇭ ⇮ ⇯ ⇰ ⇽ ⇾ ⇿