If I lived in Manhattan and never had to climb over a bridge I'd be riding an old 3 speed from the 1960s-1980s. It could be British, American, Japanese, etc. These abound on craig's list - take along a friend who knows bikes, and don't spend over $50-100. Yard sales out in the suburbs might yield a better deal. I personally have owned and loved several over the past 25 years - all free, found in the trash...many in like new condition!

These have three internal hub gears, are business-clothes friendly (sit up riding position, fenders, chain guard), built like tanks, so you can load a basket/rack on, and are fairly theft resistant so you can lock up anywhere with little worry. If they do get stolen or damaged, just go get another - if you get a month or two out of one, that more than makes up for all the subway fares you'll save.

The Raleigh Sports model is the king of these bikes, but other British names like Dunelt, Triumph, Rudge and Robin Hood are fine too. Schwinn and Columbia made them, as did Ross, Puch (Austro-Daimler), Fuji, Panasonic, etc etc

If you want utility cycling, not sportiness, go with a three-speed. Check out Harris Cyclery's web site - you can mod the gears very cheaply and it will be even easier to pedal (I threw a $25 new 24T cog on myself in about 30 minutes)