Pide
Thin pita dough covered in the toppings of your choice; we usually stuck with cheese and/or meat to be simple, and more often than not the cheese-only option ended up being our favorite. Just a fair warning, ordering a cheese pide may evoke incessant laughter from the waitstaff. I'm still not sure what that was about- was it because the English word "cheese" sounds funny? Or, as Jeremy hypothesized, do they have a chalkboard in the kitchen on which they write down the age of the oldest person who orders cheese pide, and the previous record was held by a 5-year-old? In any case, order a cheese pide, ignore the laughter, and thank me later.
Lahmacun
For some reason this reminded us of Mexican pizzas from elementary school. Flatbread is topped with things like tomato sauce, meat, lettuce, and tomatoes, then rolled up in a wrap to facilitate eating. We tried lahmacun from the stall right outside the spice market, and it had a slightly sweet tomato sauce and well-seasoned meat. It was pretty good, but it would have been better had it been warmer and fresher.
Wetburger
You'll find a bunch of places serving wetburgers in and around Taksim Square. These are the kind of things that are best eaten after a night of drinking. Although we were sober when we tried them, they were still satisfying. Basically they are small burgers covered in a smoky sweet tomato sauce and served on a bun. They're oddly delicious, and at only a few Turkish lira per burger, they're the kind of things you could eat by the dozen. Yum.
Kumpir
Like the wetburger, this probably falls under the category of late night drinking food. You can find kumpir all along Tarlabasi Boulevard adjacent to Taksim Square. Kumpir starts with a giant baked potato to which is added butter and cheese; this concoction is then mashed all together inside the potato skin. Next you can add toppings of your choice like corn, mushrooms, olives, pickles, salsa, hot sauce, and yogurt sauce, just to name a few. Maybe we just picked a bad combination of toppings, but we were not really impressed. It's a whole lot of really cheap food, and that's about all we can say for it.
Kofte
We had tried kofte elsewhere in Turkey and it was definitely underwhelming. In Istanbul we opted to try kofte at the famous Tarihi Sultanahmet Koftecisi partly for its convenient location. The kofte here was much more seasoned than it was at other places we tried in Turkey, but it was still hard to get over the fact that you're just eating straight up meat with no contrasting flavors or textures. Even though at Tarihi the kofte was served with tomatoes and hot peppers, to us it still seemed like something was missing, and a sauce would have been an incredible improvement.
Ayran
We tried this once, and that was enough. It is a very salty yogurt drink. It is so salty that my brain thought it should be a soup, except it was cold instead of hot. We gagged down a few sips in the name of science and that was all we could muster.
Balik Ekmek
Under the Galata Bridge you'll see restaurant after restaurant offering balik ekmek, or fish sandwiches. Some of the people standing outside the restaurants can be very annoying trying to get you to come inside. We opted for the first restaurant towards the southern side of the bridge, where no one was outside hassling people and the crowds hinted that it would be a good place. The fish was grilled on a flat top of sorts and in such a way that it looked like it was lightly fried; it developed a thin, golden crust on the outside. Served on a hoagie style bun with lettuce and onions, and a lemon wedge on the side, this certainly isn't fancy food. But considering it will only set you back 6 TL (about $3 US), it's definitely a delicious bargain.
Gozleme
What's not to love about thin, crepe-like pancakes stuffed with salty, creamy cheese? Absolutely nothing. Although it seemed gozleme was more prominent in other parts of Turkey, we still saw a few street vendors serving up these delicious crepes in Istanbul.
You may notice one glaring omission from our street food list: kebap. I didn't mention it because, well, Greek gyros are leaps and bounds better than Turkish kebap. But if you don't go to Greece right before you visit Turkey, then by all means, try the kebap.
Although we wish we would have had a few more days in Istanbul, I think we pretty much got the gist of the street food scene: cheap, fresh, fast, portable, and delicious!