The Digitakt from Swedish hardware boutique manufacturer Elektron is a sampler and sequencer. And by "sampler," we mean the real kind on which you can record and edit external audio -- not just load up WAV files or whatnot.
One use case of the instrument is to record an external synth riff over an existing groove and that fit it to that groove, as Elektron's Devon Hughes shows us in this video. Via the Overbridge feature, it also interfaces seamlessly with a computer and functions as an audio interface.
The overall feel of the thing is as if Scandinavians put their own take on a modern MPC, and though Elektron is beloved of electronic dance music producers, what I saw and heard suggested the Digitakt would be a very immediate and fun way to create beats, bass lines, and other riffs for any style of music. See for yourself.