Everyday Looper allows you to record 4 musical phrases - 6 on an iPad - and loop them right away. You can then mix and merge them on the fly.
Thanks to the metronome (click track) and the auto start/stop of recording (quantized recording), reachable in the tools and options menu, it's suited for two-handed instruments too.
To be noted : This app is all about recording. No pre-recorded samples are included and iPod Touch users will need an external microphone.