I usually get a break in the middle of the day and go out to eat. I need a break in the middle of the day to get out of the office.
However, life doesn’t always work like that, so I have to have provisions on those days when I can’t take a break or don’t feel like it. My staples are granola bars and microwave popcorn.
What are yours?
My situation has changed a bit, but I used to keep dry soup packets and use them with hot water from the coffee machine. I was a microwave popcorn fan, too, and I also kept almonds around for emergency food.
I travel constantly for work, back and forth between a dozen different sites. Most places offer free coffee and sodas, and if I time my visits right, I can sometimes even score free lunches from leftover catering from meetings. But I’ll occasionally carry Pop Tarts, crackers, or a bag of Big John’s Beef Jerky (the best jerky EVER!) in my briefcase just so I have a snack if I start to feel my blood sugar dipping.
I keep a kettle, sugar, milk and tea at work, and drink cups of tea during the day. I also keep some bagels and whipped cream cheese in the fridge. I don’t always have time to make them, but they’re there if I want them.
Oh, wow. This is going to be a list. My classroom is all of 35’ from the cafeteria, but I still keep supplies on hand for just about anything. Currently, I have in my classroom: 1 case (more or less) individual serving cups of Rice Krispies, 6 MREs, 1 TOTM, a couple dozen snacks from various MREs, 6 cans of microwavable soup, 4 cans of microwavable chili, 2 cases bottles water, 100 or so individual bags of chips (assorted), and several large containers of home-made Chex-Mix. And chocolate. Lots and lots of chocolate.
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I used to keep some pumpkin seeds available, and (like Big Bad Voodoo Lou) some Pop Tarts - but it’s gotten to the point where I don’t trust myself with them anymore.
Taking their place are some mini pretzels (great snack), and a bag of Sun Maid raisins.
LilShieste
I have Honey Lemon Ginseng Green Tea bags , a couple of Luna bars, a couple of Campbell’s Instant Noodles, some Kellogg’s Nutri-Grain bars, and I usually keep Vinta crackers and a no trans fat type cookie. My current fave is Peek Freans Bran Crunch cookies.
When my walk to/from work was 40 minutes, I’d keep yogurt or some other protein snack in the fridge to have at 3 because if I didn’t, I’d be so hungry after walking home that I’d eat whatever wasn’t nailed down as soon as I got home.
Several different microwaveable soups(Chinese Chicken and Corn, Pumpkin, Red Curry, Chicken Laksa, Tom Yum, Thai Pumpkin and Coconut) croutons, V8 fruit and vegetable juices, small packs of nuts with dried fruit. A grinder with Szechuan pepper and a bottle of Extra Hot Peri-Peri Sauce. Some assorted tea bags for use in the tearoom as the work supplied tea is crap.
I have some stashed in desk drawer, and underneath my desk I usually have box of crackers. I’ve also kept almonds (yummy) and dried cherries. Yoghurt in the office fridge, but I never eat it. I just buy it, take it to work, leave it there, then Mara throws it away after a few months. Everyone knows about my emergency crackers and they are welcome to have some. Some of the people in our office work all day without lunch, staying until well after 5 p.m. I’ve brought in things like those flat crackers and a jar of peanut butter for the kitchen for emergency no-lunch-today foods. It’s good to have something that doesn’t go stale quickly and can be eaten at room temperature on hand.
Our office also usually has cider, wine and sometimes beer in the kitchen area, for Friday night “wine and whine” sessions. We are highly disciplined, and don’t break into the tipple during the business day. 
Beef jerky, almonds, energy bars, crackers, cheese, water bottles, and a shitload of coffee.
Personally, I keep nuts in my desk drawer for snacking.
Collectively, we have a file drawer filled with those individual snack size bags of potato chips and Fritos, etc. There is one place where we order lunch from that includes those snack size bags with every order, only we don’t really want them so they go in the drawer for working-late-I-need-a-snack emergencies.
A bag of fat pretzels from the Orowheat thrift shop a few blocks from me. Filling, and low fat. I drink water in the afternoon, and free coffee in the morning.
Beef jerky and tuna fish with crackers (but I usually throw out the crackers.)
I work out by the airport where things are either oil-related industrial, car dealerships or hotels. If I don’t bring a lunch, the selection is sparse and a ways away.
My bookshelf contains 10 or so containers of ramen noodles, cans of pop and juice, and a 1/2 dozen cans of fruit.
The fridge has at least one frozen microwave meal, and my drawer contains some chocolate bars and other assorted candy, and a poptart from the previous programmer who left 3 years ago that I keep for fun. 
I once figured that if Disaster™ stuck, I could personally keep the 10 of us fed for about a day.
Both genmai and gunpowder tea, cans of tuna fish, high fiber crackers, apples, and a jar of mayo (in the company fridge). I drink the tea and eat at least one or two apples everyday and eat the tuna and crackers on Monday, Wednesday, and Friday.
Wasabi basted peas. Yum.
An assortment of 100 calorie snack bags of different stuff so when the junk food machine starts whispering my name I can ignore it, and an empty water bottle to refill at our water fountain. Before Easter break my desk was a little stressful getting several grants out so I was eating a lot of aspirin. I’ll include those as a food group as well.