Oscillators
Discrete transistor oscillators are used for low frequency oscillators (LFOs, <20Hz) in modulation effects, bias oscillators in tape delay effects (typically 40-50kHz), and clocks for high frequency (>20kHz) applications like driving bucket bridge delays.
Examples:
Low Frequency Oscillators (LFOs):
Uni Vibe
Rhodes Suitcase Tremolo
Schaller Tremolo
Bias Oscillator for Tape Effects:
Echoplex
High frequency clocks:
Boss CE-1 (Q3/4/5/6 drives BBD, op amp LFO)
Test tones:
The multivibrator is a type of circuit that can oscillate (astable multivibrator) or momentarily change states (monostable) or toggle between states (bistable or flip-flop).
Multivibrators can be used in effects for clocks (see the CE-1 BBD clock), as the electronic toggle switch for a bypass system (see the bypass article), or as frequency dividers to create "octave down" and "2 octaves down" outputs from a guitar input. It is rare to see a discrete transistor octave divider in a guitar effect (I'll keep my eye out...) but they are common in 1960s transistor organs. In the case of a transistor organ, the octave dividers need to divide until the whole keyboard is complete. This is typically five octaves or so.