Macaw is a program that implements a clip based MIDI sequencer, which drives a bunch of built in plugins, MIDI devices and VST plugins. It has a soundfont loader and wave editor.
The program is at a very mature stage and works fine for composing music and live playback of samples and plugins. The plugins and sampler and MIDI tracks can be played using the PC keyboard, or a MIDI keyboard or other MIDI controller.
Unlimited tracks in unlimited nestable track folders.
Sequence Audio and MIDI clips.
Import and export MIDI.
Import and export Audio.
MP3, OGG and WAV formats supported.
8, 16, 24 and 32 bit PCM.
Customisable sequencer theme.
Share plugins amongst multiple tracks.
Sequence to external MIDI devices.
Easy to install, good workflow...
edit : oops, I forgot the link :
Includes a modular synth built-in engine !