I play such a PC in a campaign right now. Me and the DM worked up an "urban ranger" named the City Hunter. It gave up spellcasting ability for a +2 bonus in Spot and Sense Motive. Since I am taking mostly rogue levels for Sneak Attack, this works well, though a PC taking mostly City Hunter class levels should probably receive higher Spot and Sense Motive bonuses at higher levels, to balance out with the complete loss of spell ability.
Basically, a ranger in the city would be good at spotting people out of a crowd and telling when people were on the up-and-up, but wouldn't really have any woodsy spells.
"Urban Lore" swapped for Wilderness Lore, and Track usable to follow a lead through the city, locate a gang's hideout, etc, using Urban Lore (as regular Track uses WL.)