Tiny in comparison to the Grand Bazaar, but much cheaper. Especially the jewellery, if you see something here that you like, don't wait for the Grand Bazaar expecting it to be cheaper, it won't be, so buy it here. Wander outside and find the animals, they have some very interesting sights, especially the massive bottles of leeches.
Even though as a westerner you have a target on your back at any bazaar, this is worth a looksee. We were able to buy some really nice teas and spices there for transport back to the states. What we did was find one nice seller and stay there. Buying in bulk seems to grease the wheels of industry. And be sure to stay after the deal to enjoy a cup of tea with the proprietor. It's traditional.
The Misir Carsisi (Egyptian Market), often called the Spice Market, is another top market visit in Istanbul. it is less crowded than the Grand Bazaar usually, and much smaller, being an L-shape. It is also geared primarily towards comestibles like tea, candy, spices, nuts, fruit leather, and the like. It is a great place to tatli sucuk ("sweet sausage"), which is a wonderful confection of walnuts strung together and covered in solidified pekmez, or fruit syrup. There are also vendors selling su boregi and the like.
Outside, to the east of it near the Yeni Cami, is the pet and nursery market, an interesting site of its own.
More of an everyday market than that which attracts the tourists at the grand Bazaar, the spice or Egyptian Market is a myriad of colour and aromas of exotic and unknown spices.