Naschmarkt is the most popular street market in Vienna. Active since the 16th century, you can find absolutely everything from food stalls, fruit, cheese, spices, bread, typical clothing stalls or decorative things. There are also products brought from Spain, especially fruits and vegetables. When there's good weather they open terraces where you can eat something and have a delicious Austrian beer.
If you are lucky enough to go to Vienna in November or December, you must browse through the traditional Christmas markets that are located all throughout the city. There are many, but among them are the Schönbrunn (next to the famous palace), and, above all, located in the Town Hall Square, the very center of the city (getting there is easy, both metro and tram). many different items are on sale at this market, though it is devoted mainly to Christmas decorations and to candy. Toys, books, textiles and glass are some other products that you can find (on the web you can find a list). To combat the winter cold, it's best to try a typical Punsch (without alcohol) or Glühwein (although I admit that to me either I came to like too much), and go with some sweet. It's a good place to get lost in the city, especially for the evening, when all of the stalls are open and there is an especially fun atmosphere.