Audiotool is a free (at least for now), modular audio production center inside a web browser. It’s surprisingly powerful and customizable, with features that rival those of some well-known commercial production programs.
The tools available include drum machines, sequencers, synths, mixers, a crossfader, a resampler, and many effects pedals (distortion, delay, reverb, phaser, flanger, parametric EQ, compressor, and others). They can be patched together in virtually unlimited combinations to give you the sound you’re looking for.
Audiotool also has a robust timeline tool that allows you to outline patterns, input individual notes, and automate effects.
To get started, go to audiotool.com/app and start poking around. There are some really helpful video tutorials available, as well as some (slightly less helpful) wiki entries on the various components.