Needles. For some reason it drives me crazy to pay for new needles and throw the old ones out even though they still look perfectly fine. I don't change the needle on my sewing machine nearly often enough (at least according to what I've read) because of this. I also can't ever feel that the needle is dull compared to the brand new ones.
New bobbin holders. I always seem to end up with just enough thread still on the old bobbins that I don't want to pull it off. For my sewing machine, sometimes I'll just use up each nearly empty bobbin on a scrappy project so I can refill them, but for my long arm I'm just collecting partially empty bobbins because I replace them at the beginning of a new row since I don't like running out half way through a pattern, then trying to match up just where I left off. Maybe I'll use them up on small projects some day.
Batting is crazy expensive and I always end up with leftover pieces that I tell myself I should sew together so I don't waste. Instead, I let my leftover batting take over too much space in my closet and I go buy more.