In a city campaign, there’s many times where you need to reflect enough bystanders that representing them individually just doesn’t work – a pain with minis, but also it loses the crowd effect of slowing down runners, etc.
Luckily, The Alexandrian has some crowd and mob rules I found! I’ll try these in an upcoming crowd scene and see if I want to mod them or not.
Geek Related 
Hmmm… I just skimmed his rules and they’re a little crunch heavy, but if that’s what you’re looking for then you’ve hit the nail on the head. The base DCs and attack bonuses are a little low, in my opinion. To be honest, crowds will be a problem for high level characters as well as low level characters, so you’d have to scale them appropriately. But I’m more into 4E anyway, so I’m thinking of a different system that I’d use when I read his ideas.
A good jumping off point though.