Taxi fares in Sofia were regulated in May 2011 and a maximum fare imposed. This is currently 1.30 leva daytime and 1.60 leva nighttime. Fares are displayed clearly on the back window of the cab as well as on the dashboard. All taxis are yellow and are generally fairly new, well-kept vehicles, though you may occasionally get the odd unkempt one. Smoking is prohibited but despite this you will find that some Sofia taxi drivers smoke while waiting for a customer; if it bothers you then ask them to put out their cigarette. All taxis in Bulgaria should have the name and contact number of the company clearly displayed on the dashboard as well as the ID number of the driver, enabling you to make a complaint should you wish to.
There are some useful apps you can use such as Taxi Me and Yellow Taxi 9119.
Unfortunately tourists (and sometimes even locals) continue being ripped off by rogue taxi drivers, most  usually hanging around at the airport, train station, hotels and even hospitals. They often display logos very similar to the popular OK Taxi. The best thing is to always call for a cab or use one of the apps available.