If you've ever been curious about making beats and basslines, especially for electronic music, RD3 Groovebox ($4.85; free trial) offers a cheap way to experiment with that process, putting a beat machine and a synth in your pocket.
RD3 has three main windows: Mixer, Bassline, and Beats. Bassline is a real-time synthesizer, complete with a virtual keyboard, a sixteen-step sequencer, and a handful of effects and other ways to alter the sound. The creators modeled it closely after the legendary Roland 303, and it does a decent impression.
The sounds it creates are high quality. Since it utilizes multitouch, you can alter several parameters at once; it also has a follow key, so you can just play along on the keys and that will be recorded into the pattern. Unfortunately, when using the follow key I found that the timing wasn't quite on point, due to a small amount of lag. It was usually pretty close, though, and you can always go back and edit.
The Beats window is a drum machine. You get eight different drum kits (including 808, 909, 606, RZ1, and DMX) to choose from, which is really cool. Once you've selected your kit, the Beats window consists of six different drum channels (one for every drum in the kit) with separate punch controls for each. It also has a sixteen-step sequencer. The drums all sound good, but if you want to hear the bass you'll have to plug in some nice headphones or external speakers; a phone's speaker is just too small to accurately replicate low tones.
The Mixer window is where you put everything together. You start and stop playback, set the tempo, adjust volume for the bass or beats, and add distortion to the bassline to funk it up. You'll notice that you can create four different bass patterns and four different beat patterns per song. From the mixer window you can choose which pattern plays, on the fly; alternatively you can make them all loop or select them to play at random.
Once you've created songs, you can save them--which is a good thing, since a decent one will take you a while to create. The saving procedure has one small annoyance, though, in that when you press 'Save' it always asks you what to call the file (just as when you click 'Save As...' on your computer). Most often I wanted to save the file with the same name, but to do that you have to remember what you named it. Having both a 'Save' option and a 'Save As...' option would be nice.
As cool as this program is, a couple of issues hold it back. The main thing it's lacking is an integrated recording program. You can save the song, as in the patterns, but you have no method to convert it into an MP3 that you can play later. The only way to do so would be to use the audio-out on your phone and connect it to an audio-in on another device and record it. Who wants to do that? A simple, built-in recorder would be a killer addition--without it, the app ends up feeling somewhat unfinished. The other problem is that RD3 doesn't multitask very well. If you switch away from the window, the music stops. It's not the end of the world, but I'd like to see that fixed.
Overall, RD3 is nicely put together; it also sounds good and works more or less as advertised. If the creators add a way to do a live recording of your song (as you DJ it), it will be a whole lot better. As is, it's an amusing thing to tinker with, and a decent (and inexpensive) introduction to beat machines and sequencer-based music production.