A MIDI arpeggiator/sequencer device from Ian Dicke for arithmetic progression (10$). There´s more good stuff (also free) on his site.
RandARP is a free VST plugin arpeggiator with many randomization features, allowing incoming notes or chords to be manipulated in various rhythmic ways.
Random Riff Generator is a midi tool by Mario Nieto that generates sequences for you. This is the free version.