I'm becoming more and more fascinated by graphical, software-based music composition tools. A well-known, simple example is ToneMatrix, a pentatonic step sequencer (if you've never played with this before, you're welcome).
If you find that one interesting, give these a try (flash plug-in required, sorry):
- Online Sequencer: straight-forward, most resembles traditional organization of musical ideas.
- Drumbot: a bunch of cool tools: several drum sequencers, chord charts for discovery and composition, practice tools, and more.
- Otomata: a generative musical sequencer.
- Seaquence: my personal favorite, Seaquence adopts a biological metaphor, allowing you to create and combine musical 'lifeforms' that will then interact, resulting in unpredictably evolving compositions.