GRAPH THEORY: Find your way through 61 short music fragments and explore a composition for solo violin: http://www.turbulence.org/Works/graphtheory/
OTOMATA: Plugged string sound in a matrix. The web application contains a download option which promotes the import of your composition into your favorite daw as a wav-File: http://www.earslap.com/projectslab/otomata?q=0n0i6i Otomata is a generative sequencer. It employs a cellular automaton and sounds great! ToneMatrix on http://lab.andre-michelle.com/tonematrix is a rather simple sequencer but worth to check it out.
EUCLIDEAN SEQUENCER: It’s just not a web toy, but it is funny too. An explaination can be found here. The sequencer was programmed by Wouter Hisschemöller : http://www.hisschemoller.com/2011/euclidean-midi-patterns/
A large collection of music toys can be found on the page of Andre Michelle with interesting concepts of sequencers. My favorite is “Pulsate”: http://lab.andre-michelle.com/pulsate Place at least 2 circles on the desk and they will grow until they collide. Depending on the dimension of the circles they generate a tonal sound during the collision. A similar app is available on http://www.earslap.com/projectslab/circuli . Indeed an awesome generative music generator.