For an alternative to the excellent but pricey Bose and Sony noise cancellers, I would like to recommend the TaoTronics line. I picked up a pair of in-ear buds on sale and was pleasantly surprised at how well they worked and how good they sounded. They may not be as good as Bose, but worked well enough, and at a significant price delta, to be in my travel kit.
No link because the ones I have are no longer made, but search the company name on Amazon.