Filtatron is pretty good, there is also reactable which has audio input capabilities, its quite nice.

theyve made ones for guitar cables, and i have a converter for guitar cables. a few weeks ago a friend of mine was jamming in our schools hallways with his guitar and his ipod, and there was zero latency. i imagine with I/O capabilities they could make a similar app for synths/FX.

That's the IK Multimedia one right? The problem with it isn't so much the latency but the frequency response, it uses the microphone input which is meant to be used with the iPhone headset, the spectrum ends up being far from flat and rolls off a lot of bass and a bit of treble. This is ok for its intended purpose (hands free kit) and is acceptable for guitar, but you run into problems when you use it as a general purpose fx processor, it's not exactly ideal. That and the input is mono - if you wanted stereo processing you're SOL.

Interestingly enough it looks like if you have any of the new generation iPhones/iPod Touches or an iPad there's no line input on the dock connector anyway there's hope if you have an iPad, once some audio interfaces become supported on the camera connection kit, but until then ...
http://blog.faberacoustical.com/2010/ip … mitations/

this looks promising...ill probably homebrew the adapter because the irig one costs $40

I'm gonna try this -
http://www.cultofmac.com/plug-your-guit … irig/68113

TouchOsc is really cool as well, it's an awesome controller. It does require a computer however.

The problem with all the fx things available is there's no interfaces available for the i-devices with both line input and line output, at least not commercially. It basically means there's no hi fidelity way of getting sound in and out of the things. You can buy one of these  http://www.chargeconverter.com/shop/connector.htm and build your own of course but that's the only realistic option available.

Edit: Let me elaborate on OSC. It's a network protocol for sending MIDI signals bascally. So you can touch an XY interface on your IPOD, and have it control, say a delay effect in Live, or FL, or Cubase.

edit - the mic pics up, so you could likely use any of the in/out devices out there to use it.