Unsalted cashews (for which my wife thinks I'm a bit of freak). I also bring, as a treat, little snack packs of fabulous Greek yogurt that have honey in a separate compartment that can be folded over to add the honey to the yogurt. It certainly keeps me going when I have it around 3 pm.

I also bring a multi-part lunch and eat a section of it when I first get hungry, then wait until I'm hungry again to eat the next section of it. It spaces the food out over the day nicely. But then I rarely leave my desk to have lunch.