<sub>small text</sub> or <sup>small text</sup>

First one gives you chemical equations (like C₈H₁₀N₄O₂) and the second one gives you citations (thing you're citing^{[index number]} and exponentiation (bⁿ).

Or in our abuse of it, stuff that's not important but we want to say it anyway or image captions (until everything supports caption tags) XD