Omikron (1999) was a great game. It was one of the first open world games with real 3D graphics. You could ride a taxi around the city, enter buildings, etc. Of course GTA3 (2001) took it to the next level, but in 1999 GTA still was 2D: https://en.wikipedia.org/wiki/Grand_Theft_Auto_2 .
$10 on Steam as well: http://store.steampowered.com/app/243000/
One of my favorite games, however I recommend playing it on Dreamcast if that's an option, as the PC controls are a little painful to work with.