You would be hard pressed to find a more organized and corruption-free country than Sweden... except, for some strange reason, when it comes to taxis. Taxis aren't regulated in Sweden so prices can vary wildly from company to company. As long as the prices (fixed rates, SEK per kilometer or minute) are displayed somewhere within the car then the company can determine their own price scale. In other words, a company can legally charge double what another company does for the same trip from point A to point B. 90% of taxis in Stockholm belong to the larger companies and have similar (and reasonable) prices. Stockholmers know exactly which taxis to take and which to avoid, so these smaller taxi companies end up preying on visitors who don't know better. But, as long as you follow three simple rules you will avoid any trouble!
Stick to the larger taxi companies. Taxi Stockholm is my favorite, otherwise there are a few other good companies... Taxi Kurir, Taxi 020, Airport Cab, Taxi Transfer and Top Cab.
Avoid hailing a taxi on the street. Always have a hotel, restaurant, store or museum call you a taxi. That way you will be sure to get a good company. At the Rival Hotel we call Taxi Stockholm unless a guest asks for a different company.
Taxis are all required to have a sticker in their passeger, side window with a unit price based on a 10km, 15 minute journey. This unit price should be between 290 and 390kr for a good taxi. Anything above that will be expensive!
If you choose a taxi from a taxi line/queue then remember that you aren't obliged to choose the first car in line, you can choose the taxi company you want... even if the car is further back in line. At some locations, like the Arlanda airport, each of the large companies have their own line marked with their name. The riskier taxis have their own line marked "other".