FWIW there is a http://taxime.to/ http://taxime.to/ in Bulgaria, which has a reputation system and was embraced by existing taxi companies - you get a well-rank

FWIW there is a http://taxime.to/ http://taxime.to/ in Bulgaria, which has a reputation system and was embraced by existing taxi companies - you get a well-ranked driver that is licensed and insured, with overall quality much higher compared to random taxis in the country.