I email my ideas to myself, and label them "Project Idea". I also include several tags in the subject in order to find them and filter them easily later on (i.e. game music learning).
Game music learning sounds like a cool domain for ideas. Do explain?
It was just an example of tags I may include in the email subject, but I've developed a music learning game in the past [1], which I'm currently recoding from scratch for a music teacher who needs it for his students. I'm writing the remake as a web application, this time coded in Amber Smalltalk.