MIDI is awesome! I recently bought an M-Audio Ozonic keyboard/recording interface and kill a lot of time by writing music or mixing up some cover tunes using MIDI. For a program I use FL Studio, its a pretty basic program and easy to use. You can also use VST plugins if you wanna change up your drum sound or looking for a certain kind of synth sound. You can fully edit all parts of the MIDI track as well and replace instruments with different sounds or instruments. I have a wicked miniMOOG plugin as well as "The Drumkit from Hell" (ez drummer) plugin.
http://flstudio.image-line.com/documents/what.html
I know its not free but I figured its worth a look.