I usually buy them a gift card to a store I know they like, so that they can use the card to buy gifts for other people. Or, if they see something they like and it is so new to the market that they don't already have it, they can buy it for themselves.
I find out what kind of activities they enjoy and buy them a certificate for that activity. For example, I know my parents love Cirque du Soleil, so I bought them tickets to see Cirque du Soleil's "Volta" event, which is at AT&T Park through the end of January. I also got them a gift certificate for a pre-performance dinner in the city because, you know, they're my parents and all.
I find out whether they volunteer with — or are otherwise involved with — a charitable organization, and then I make a monetary donation in their honor.
I put my wallet away and I make them a handmade gift. Last year I made beautiful Christmas ornaments for people's trees because even someone who already has everything has room for another ornament on their tree. Plus, it's like a gift that keeps on giving because they can put it on their tree for years and years to come.
Christmas cookies! For people that don't need or want any material things, but who I'd like to give a gift to, I bake Christmas cookies. I make several different varieties and decorate them all exquisitely. When the recipients see how much time and care I put into my baked goods, they feel very appreciated and loved.