"That raise actually might not be as good as it looks. The extra money is nice, but it could very well bump you into the next tax bracket, possibly leaving you with less money than you had before the raise."This commenter shows that within our tax system you always make more money when you get a raise, you just might have a higher marginal tax rate on the extra salary, but you never make less net income. USA Today fixed it.
An even more complete discussion of marginal tax rates can be found here.
All that being said when you add in the rest of the tax code with earned income tax credits, welfare and deductions the net income vs. starting income can be very different at low incomes. This author seems to think that it provides a disincentive to work, whereas I see it as society's way of providing an income above the poverty line for families until they begin making more than that. The lowest net income seems to hover around $35 to $40 thousand a year.