I was going to suggest beans as well. I'm always at my hungriest in the afternoon, so I tend to eat a larger lunch and go easy at dinner. I love bringing chili, which keeps me full, and bean salads. Falafel is another good one. Thicker soups, like pea soup, also work well. Sometimes keeping very hydrated will help stave off hunger, so keeping a lot of water (or hot tea/iced tea) will help. Also try adding avocado or nuts to your lunch, since it's important to eat enough fat to keep you full.

If all of that fails, keep Lara bars with you. They're basically nut and dried fruit bars. I usually keep one on me wherever I go. They're a little expensive, but you can also make your own very easily if you have a food processor; there's several recipes for homemade bars floating around the web.

The suggestions for staying full longer are good, but some people find they can eat less and feel more satisfied by eating several smaller meals and snacks throughout the day.

I like to take healthy muffins and quick bread slices with me to work. This summer, we've been all about the beet muffins:
http://strawberrypepper.com/2010/07/09/beet-peach-muffins/
And this strawberry bread makes a lovely portable snack too:
http://strawberrypepper.com/2010/06/25/heavenly-strawberry-bread/

Hearty grain-based salads leftover from the previous night's meal can be great, and they don't get wilted when you pack them the way a lettuce & greens salad would. For example, this quinoa salad:
http://strawberrypepper.com/2010/07/03/quinoa-mango-salad/
or this pesto potato and snap pea salad:
http://strawberrypepper.com/2010/07/01/peas-and-potatoes-in%C2%A0parsley%C2%A0pesto/

i love snacking on homemade hummus and pita chips/veggies at work and typically feel satisfied afterward!

I would do a sweet potato salad of some kind. Sweet potato is incredibly filling and has a low glycemic index. You can prepare a large batch of such a salad and eat it over the course of two or three days.

One great sweet potato salad recipe is on page 54 of one of Claudia Roden's cookbooks, entitled "Arabesque". It's a Moroccan recipe that includes ginger, cumin, onions, lemon juice, olive oil, and parsley and green olives.

Although sweet potato has a lot of sugars, it also has a lot of fiber which drastically slows the absorption of those sugars by your body. It's loaded with other great nutrients too.

http://nutritiondata.self.com/facts/vegetables-and-vegetable-products/2668/2

It is also important to start your day off with foods that have a low glycemic index / low glycemic load.

Another option is a greek style salad made with fresh tomatoes, cucumbers, black olives (the inexpensive sliced canned kind), red onion, and maybe some capers. I like to cut everything into small, bite-size chunks, not like those huge discs of cucumber and rings of onion that you get in chain restaurants.

Place the dressing in a separate, smaller container. The dressing should include: olive oil, balsamic vinegar, lemon juice, crumbled feta, minced parsley, black pepper, salt and lots of dried oregano. In a pinch you can substitute some other vinegar such as sherry vinegar for the balsamic, some pre-grated parmesan for the feta, and forgo the parsley entirely. Feta is really delicious though, and there is really no great substitute.

Add a cold grilled chicken leg to your lunchbox along with the greek tomato and cucumber salad, and you have a healthy lunch with a little fat and salt to satisfy your cravings, all with a very low glycemic index. I always find that eating a salad of fresh tomatoes and cucumbers gives me real energy boost.

Oh, sorry for the chicken leg part. I just saw that you requested something vegetarian. Maybe instead of the chicken leg you could do a container of oven roasted tofu with a nice glaze. You'd get a lot of the same crispy/chewy texture and protein that I was going for with the chicken leg.

You could also bring along some spicy oven roasted chickpeas, there are plenty of recipes for this type of thing online. Usually they involve oil, salt, and spices like cumin, paprika etc.

Cold salads are really a great thing to bring along for lunch though, and I highly recommend them. You can prepare large batches that will last for days, and your body will thank you.

Two more snack options that would do well refrigerated:

Two or three dates, stuffed with cream cheese and topped with an almond or chopped walnuts.

An apple, some grapes, and some sliced cheese like cheddar or gouda.

Maybe some grown-up "ants on a log"! Also, I sometimes make (or buy) oatcakes and eat them at work, sometimes plain and sometimes with cheese or a veggie spread. Vegetable spread is a new revelation for me: I read about it in Mark Bittman's latest book and it's great: just cook any veggie, say sweet potatoes, and then puree with a few drops of water if needed and season. Genius! What a great way to eat more veg, as well.

Can someone tell me what the snack in the picture is? The credit just says it was taken by Faith, but not the subject. Thanks!

@roseslaw I think those are peanut butter balls

http://www.thekitchn.com/thekitchn/candy/weekend-snack-milk-and-honey-peanut-butter-balls-080885