I've been looking desperately for somekind of musicial sketchpad for my Q, with little avail. Today I ran across a sweet little app called MiniMIXA(
http://www.minimixa.com) made by the Tao group....and it blows everything on the mobile market away!
MiniMIXA is what you get when you cross breed Ableton Live, Reaktor and the Motorola Q. It comes in a free SE version and a $20 pro version. The SE version is fine for twiddling around, but MiniMIXA really shines with it's pro version. The free version will load up loops(in Wav, MP3, Ogg) and MIDI data and allow you to sequence it both in a grid view or linear sequencer view - yes...just like Ableton Live. The pro version takes it a hundred steps further. You get a complete software synthesizer with 4 channels of soft synth yumminess. The synth engine has some gritty attitude and a personality all it's own. You can create complex effects chains, create your own wavetables and do on the fly sampling using the Q's microphone.

You can also e-mail your creation as an SPMIDI (MIDI for phones) or as a mixed down WAV file. So you can create your own ringtones.
There are cons though...there always are. MiniMIXA does not scale correctly on the Q due to their skinning engine. The default skin only covers half the Q's screen (if you download it...please don't bother Tim about it...I've already made him aware of it and his development team is working on it). The second con is MIDI support. The program does not have it's own piano roll or step sequencer. You have to create your MIDI phrases in a real DAW (like Sonar, Cubase or Ableton Live) and import them onto the Q. From there you can apply soft-synth patches on your MIDI clips. The interface might be a bit confusing at first, especially if you've never used a MIDI sequencer before, but after 15 minutes of exploring it should come to you very easily. This program doesn't replace a good DAW like Cubase or Ableton Live...but definately compliments them (especially Ableton Live)
MiniMIXA -
http://www.minimixa.com
FreeSound -
http://freesound.iua.upf.edu/ - If you just want to play with the free version, here is a massive library of free loops and sounds you can use.