We are going to St. Petersburg for a week and we would like to buy a sim card for our Samsung smartphone, so that we can use phone and Waze. I looked for russian sim-cards in Google and all I found were plans with daily or monthly payments, that probably require a credit-card. Is there a way to buy a pre-paid sim card without a credit card? If so, what is the best place to buy - in the airport or on street shops?
Yep, there are plenty of shops in Russia selling prepaid SIM cards. Most large airports have at least one (and SVO had several), or you can buy one in the city. Just walk in a shop showing up mobile operator logos (MTC, Beeline etc). Those shops sell several prepaid plans, and will both sell you a SIM card and top it off.
By Russian law you are required to produce a passport when buying a SIM card. This is sporadically enforced in airports and better shops, but smaller shops next to subway would sell you anyway. There are also people (re)selling SIM cards in some underground passages; I bought from one and had no issues with it.
When buying it, you can also top it off at the same place AND ask to turn off all the "extra services" they usually keep on. Some of those services (like ringtone music) can easily cost up to a dollar a day, and can eat up your deposit fast.