The Compose Key

My own use, both in KDE and Windows (with WinCompose):

The 'Caps lock' key becomes the Compose key (actual caps lock is achieved by simultaneous left shift + right shift).

Compose, *, <letter> produces a relevant Greek letter (a -> α, S -> Σ, and so on).

Compose, (, <number>, ) puts the number in a circle: ③

Compose, s, s results in ß (Compose, S, S gives ẞ).

Compose, Compose, t, h, m, u, p results in (THuMb UP).