The Twin-T Oscillator is a form of Sine Wave Oscillator with
an interesting topology. The Twin-T Oscillator uses two
"T" RC circuits operated in parallel. One circuit is an R-C-R "T"
which acts as a low-pass filter. The second circuit is a C-R-C "T"
which operates as a high-pass filter. The result is a much cleaner
sine wave than the typical 555 based relaxation oscillator.
Some typical circuits:
A Phase Shift Code Practice Oscillator from qrp.pops.net
The MorseBox from VK3YE
Discover Circuits Twin-T Oscillator PDF
Twin-T Audio Oscillator from G0XAN
Twin-T Audio Sine Wave Oscillator from Art's Theremin Page
An Audio Oscillator for Transmitting MCW
is a project that uses a Twin-T oscillator for modulating
a 2 Meter HT with Modulated CW. This enables Technician
license holders to practice Morse Code even if they do not have
an HF rig.
There are at least two commercial kits that use a Twin-T oscillator:
The Jackson Harbor Shaped CPO kit - Schematic PDF
The Morse Express MX T-Tone Code Practice Oscillator Kit
The Morse Express page for the kit has two nice Morse samples of the CPO output.