My favorite free on-line step sequencer is Step Seq on Kongregate. It looks great, it has an intuitive interface, and it is powerful. You can write drum parts, melodies, accompaniment, and switch between sections easily. Even better, the tones are good and are great at getting you into rhythm and wanting to play along. Unfortunately, it can be quite buggy. The animation engine it uses, Unity, is flash based and is slowly getting excommunicated from the world-wide web. You may still be able to get it to run without any issues, so I recommend trying it. It is super fun and full of educational promise. If it doesn’t work for you, try a different browser, and if it still doesn’t work, see you can get it to work by following the instructions in this video.
A more reliable, but much less powerful drum machine you can also try Monkey Machine. Another online step sequencer is onlinesequencer.net. This one is the most powerful of all, but has a rather awkward interface and is not very intuitive.
For iPad or iPhone, try the Rhythm Lab app and the Rhythm Calculator app (A similar android app is Rhythm Maker). Android also has a very fun app called Miniseq. Step Seq is available for iPhone and on Android it is called called Sequencer Music Maker. It could be amazingly good, but the ads really hobble it. I find I have to turn on Airplane Mode before I start up the app. I also recommend the PolyNome iPhone app.