You can for instance use mouse movement as seed for random numbers, read out time and mouse position whenever the onmousemove event happens, feed that data to a whitening function and you will have some first class random at hand. Though do make sure that user has moved the mouse sufficiently before you use the data.

Edit: I have myself played a bit with the concept by making a password generator, I wouldn't guarantee that my whitening function is flawless, but being constantly reseeded I'm pretty sure that it's plenty for the job: ebusiness.hopto.org/generator.htm

Edit2: It now sort of works with smartphones, but only by disabling touch functionality while the entropy is gathered. Android won't work properly any other way.