Urban Disguise from Think Tank, but they're more expensive than Domke. Somewhat less expensive than Think Tank, but similarly inconspicuous is the Domke F-803. They all look like small laptop briefcases. Some people think it's not a good idea to carry something that looks like a laptop case, but they're pretty much ubiquitous in New York, so something that looks like a laptop case is less conspicuous than something that looks like a camera bag around here.

Crumpler's also nice for bags that look like messenger bags.

You might troll for bags on eBay. Sometimes you can find good deals on bags that aren't in bad shape.