Polar night. I've posted a few times about this. I had it after my braces came off-£140 through my orthodontist (so they obviously recommend it) it's much cheaper on line though. Very easy to use, fantastic results (my teeth are now whiter than the whites of my eyes) in 10 days and I didn't get any sensitivity.
If you do use it be sure not to over load the trays-you really do only need a tiny tiny dot of gel in each tooth mould.