A now free cross-platform MIDI sequencer surfaced as a VST/AU/etc. plugin. More info & download.
Giada is a free sequencer environment for handling your samples and loops. VSTs can be loaded as well. Here´s a demo vid:
The Music and Sound Design Platform (also referred to as Music_SDP and MSDP) is free and open-source software built for electronic and electroacoustic music performance, production, improvisation, and experimentation. Music_SDP is a DAPE, or Digital Audio Performance Environment, functioning similarly to a DAW (Digital Audio Workstation), but designed with a special modular structure built for live performances. MSDP provides a non-linear, infinitely expandable, totally hackable, easy-to-use interface for sound design that encourages and rewards exploration and experimentation. It is a great platform for beginners and professionals alike, and provides all the tooling and control needed to make wild new sounds, build complex sonic projects, and execute flawless performances.
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.