There are two online music research experiments going on right now. Henkjan Honing, a very nice and smart Dutch music cognitionist, is looking for volunteers to judge the quality of 28 audio files. Participants mark whether the file has been digitally manipulated. The whole experiment takes less than 15 minutes, and is rather fun.
Victor Adan, a graduate student at MIT's Media Lab, is seeking subjects for an experiment on contemporary piano music. Participants rate excerpts on similarity, preference, and complexity. Victor estimates the experiment to take about 50 minutes. I haven't had time to do this yet, but it sounds like an interesting project. And you owe it to science!