As we've been saying in our Slabcake broadcasts, it's very easy these days to play around with some quite complicated musical ideas without all that tedious mucking about with notation and musical theory. Everyone (unless you're clinically amusical, which is very rare) has an innate sense of music: we can all speak and listen to the language even if we're not trained in the written form, the grammar and the practice of writing. As they used to say on ArtAttack!, try it yerself. They're all free to play with.
Tonematrix
Like a basic, online version of the Tenori-on, it's a simply grid full of squares each of which is a note. Click on them to light each one up and watch your patterns turn into music. Although it looks quite random, you don't get to control tempo, time signature, scale or mode but it's fun to play with.
http://lab.andre-michelle.com/tonematrix
Pulsate
Moving on from Tonematrix to something more abstract. Don't want to give away too many clues as half the fun of this is exploring how it works. To get started, just click quickly on two random places in the square and see what happens...
http://lab.andre-michelle.com/pulsate
Musical Dice Game (Musikalisches Wurfelspiel)
The most famous example of aleatory composition, i.e. music determined by the roll of dice. Attributed to Wolfgang Amadeus Mozart, although it was actually published a few months after Mozart's death in 1791, by his publisher Nikolaus Simrock apparently based on notes left by the composer. It doesn't particularly matter whether it really Mozart's creation: the idea of the musical dice game had been around for many years and Mozart's wasn't. Perhaps it was merely a clever bit of branding by Simrock. A roll of dice is used to select small sections of written music which can then be patched together to create a single musical piece. Another version also attributed to Mozart uses letters instead of numbers and invites the “player” to produce the composition derived from the spelling of his or her name. It is therefore theoretically possible to translate the script of an edition of “Homes Under the Hammer” into a composition by Mozart.
http://sunsite.univie.ac.at/Mozart/dice/
(Note: Cataloguers do not credit each of these as individual compositions so if it was an attempt by Mozart to claim to be more prolific than Bach, it didn't work.)
Bhudda Machine Wall
The Bhudda Machine is a lovely simple idea. It's a little box which plays soothing Chinese music. There's an online version where you can play around with up to 21 boxes all at the same time and produce your own blend of calming ethereal oddness. (And well done to the guys who made it for leaving the sounds under Creative Commons open copyright.)
http://aux.zendesk.com/wall 