Christian Budde has released Simple Chorus, a free plug-in that was originally created as part of Christian's Delphi ASIO & VST Open Source Project. It is one of several examples and the full source code is available.
The effect itself is nothing more than a simple chorus implementation with up to 8 stages, speed 0.01-10 Hz and a depth of several octaves (frequency dependent). A mix parameter controls the balance of the sound (0% = dry, 100% = wet). Additionally a drift parameter controls the stereo width and the thickness of the sound.
Simple Chorus is available as a free VST effect plug-in for Windows.