Gameswithwords is a great site that has been set up and run by a number of researchers who are studying aspects of language. In order to generate larger sample sizes, they realised that they could recruit more participants by making their experiments accessible online rather than having to put up ads on noticeboards and convince people to come into universities to partake in the study.
The tasks don't take much time to complete and they make the results available on their blog when the data has been collated and interpreted. They also give you the option of being emailed with the results so for those interested in research and want first-hand experience in what it's like to take part, here's your chance!
Check out some of the studies and feel free to post any thoughts or feedback here.