I voted for 'Dice' and 'Rulebooks'. In hindsight, I should have also added 'Other'.
I had a DM buy a set of gorgeous tungsten dice for all the players in a long-running campaign. It was a marvelous gift, and whenever I look at those dice, I remember the good times we had in that game.
Early in my DM'ing career, 3rd Edition had just come out. I was the DM for our group, and we were playing 2nd Edition. We were between games, and one of my players took it upon himself to buy the three core books for me, which was a stunning display of generosity. We were all high school students at the time, so $100 was a considerable amount of money.
I also received a mug from my players with an engraving 'World's Greatest DM'. It was an unexpected thing, right in the middle of a campaign, but it really made me feel all warm and fuzzy. A small token of gratitude like that goes a long way.
Over the years, we've started a tradition of giving out mugs to DMs after they 'earn their wings', so to speak, by running a full-fledged campaign. We gave my buddy 'Epic DM' after he actually ran a 30th level 3.5 game that saw us reach 34th level. Another friend earned 'Completionist DM' after she impressively finished her third campaign (she also loves to score 100% in video games). I'm sure there will be more in the future.