Harmony
A few of the different stylistic approaches available and utilised:
Monody and Drones
Canon
Counterpoint
Diatonic Modes and other Native Systems, including exotic scales and PC Sets
Partimento and Galant Schemata
Common Practices (e.g. Tertian, suspensions, preparations, ornaments)
Pivots, Modulations and Tonicizations
Constant Structure (a.k.a. chord planing, or parallel movement)
Secondary Functions
Borrowed Chords
Substitutions and Alterations
Multi Tonic System
Neo-Riemannian Theory, including parsimonious transitions
Axis System
12 Tone (Dodecaphonic)
Mutually Exclusive Triads, Tetrads, and Complementaries (Duodecimal, or limited within specified PC Sets)
Serialism
Quartal, Quintal and other non-Tertian systems
Clusters
Tintinnabuli
Microtonality
Polytonality
Temperament Exoticism (e.g. 55-Div, 31-EDO, Bohlen-Pierce)
Blues (cf. Constant Structure, but also Microtonality)