Is there such thing as an active-low, logic level mosfet?
I need to make a logic level switch that will turn on when the input
  signal is 0V (active low)


You can use a suitable P channel MOSFET for this: -



Picture from here.

So, if the logic input is high, the P channel MOSFET is turned off and the load is disconnected from the +5 volts supply. If the logic input is low then the MOSFET activates and the load is connected to the 5 volt supply.

You can also use logic level analogue switches but watch out for limitations in on-resistance however, if you require really low leakage current (device deactivated) then an analogue switch will probably be better than a single MOSFET in many circumstances. ADG802 Example: -



The above has an on-resistance of 0.25 ohm and operates from 1.8 volts to 5.5 volts. All the usual vendors (TI, Maxim etc.) will have equivalent versions of this device.