1. Measuring tape. It doesn't have to be fancy, just flexible, fairly long, and marked in both inches and centimeters.
2. Yarn winder. Always rewind skeined yarn and avoid frustration! Our machines use up yarn so fast that they need yarn that feeds quickly, smoothly, and evenly. My favorite yarn winder of all time is the jumbo hand winder, which holds a whole pound of yarn.
3. Sturdy knitting stand. Article on using a good stand, here.
4. Wipe-off markers (I like Crayola washable markers and also Vis-à-Vis washable markers. Avoid permanent markers.
5. Ravel cord. You might have gotten a couple of yards of ravel cord with your knitting machine, and it was plain white. Supplement that! I love Omega's Artiste nylon cord, which I find at Hobby Lobby (buy hundreds of yards cheap, get whatever color you want, and use all you need).
6. A sheet, for covering your machine. You want a piece of fabric that blocks light but breathes.
7. A vacuum, for cleaning your machine.
8. A spiral notebook to keep near the machine and write down notes and ideas.
9. A pocket calculator, for working out the stitches and rows for shape.
10. Oil for lubricating your machine. Beware: you must use very light oil that will not harm plastic. I like Hoppe's Elite Gun Oil.