I ported the ThNdl GLSL tutorials into a Quartz Composer patch from a processing sketch made by processing forum user, sableRaph. The tutorials are some of the best I've seen out there and make it great place to get started with GLSL! Just comment and uncomment code in the GLSL patch and it updates in the viewer.
QC Patch: http://bit.ly/10Yyos3
Github repo: https://github.com/rybotron/thndl-Quart ... -tutorials
Original Processing sketch from sableRaph:
http://forum.processing.org/topic/thndl ... -with-glsl