Macaw - Free VST sequencer with Audio (open source)
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.
A list of features:
Multitrack MIDI Sequencer.
Clip based sequencer.
Sampler - load soundfonts, edit soundfonts.
Synth - load plugins.
Load VST plugins.
Import MP3, Ogg, AIFF, Voc, WAV etc...
Export whole song to MP3, OGG or make samples per key or split to the soundfont editor.