One good example is Orb Plugins. This collection of plugins includes Orb Producer 3.0, which is geared specifically for creating chord progressions, basslines and arpeggios. It's also got an intuitive interface and works with most digital audio workstations (DAWs) so it's a good all-purpose music production tool.
If you want more general music generation abilities, Beat Shaper offers a lot more, including beat, bassline and melody generation. It's designed to help you be more creative and productive by generating material based on your style, but it's got full creative control and ownership.