Probably the best gift I have received in the last ten years as a missionary was my Kindle. I love to read, and I love to read in English. To have instant access to books has brought such joy to my life after many years of not being able to find books, having to wait until a visitor came or another missionary was available to do a book exchange.
Another great gift to go along with that is an Amazon gift card. My aunt always gives us a gift card for Christmas, and this usually lasts us almost the whole year as we search for bargains on kindle books and try to stretch out the money. My mom also gave me her library card information, as many libraries now allow you to check out books on kindle for two weeks at a time.
Adding to my list of good gifts, a couple of years ago my friend gave me an ipod. Since I live on support I find it difficult to justify spending supporters' money on luxury items such as that. When someone gives you a luxury item it is such a great thing! I have used my ipod almost daily, and am thankful to have access to songs in English as well, since the radio stations don't carry many songs like that.
If you are looking for some good gifts for missionary friends, consider gift cards to amazon and itunes, starbucks (which takes international gift cards), and fun tech items that they would never be able to purchase or would feel guilty buying for themselves (tablets, game systems, mp3 players, etc.)