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Along the same lines as Lemur, there is also TouchOSC. If you really like the idea of making your own stuff to use, there is also an iOS port/implementation of PureData, which is very similar to Cycling74's Max/MSP.
http://www.mobmuplat.com/ - iOS port
https://play.google.com/store/apps/details?id=org.mpd&hl=en - Android [mPD]