Mosfet type to drive a 100A DC motor using pwm
You may want an IGBT for this; the way to use them is very similar to a MOSFET but they tend to be much more sturdy devices (for instance, all IGBTs rated at >100A seem to have voltage ratings >250V as well!).  This is important because of inductive kick that is quite easy to get with motor loads.  MOSFETs are in theory protected by their body diodes, but in practice need a snubber circuit as well for reliability.

An example of a suitable IGBT is AOK50B60D1, a 100A, 600V part for $2.13 @ 1k.  It is rated at 300W, but would likely dissipate quite a bit less in your case.

If you do want a MOSFET, try something like IRFB4115PBF, 150V, 104A, $1.87; or IPP048N12N3.

EDIT

A bit more detail: IGBT vs MOSFET: Choose Wisely, by IRF


  IGBTs have been the preferred device under these conditions:
  
  
  Low duty cycle
  Low frequency (&lt;20kHz) 
  Narrow or small line or load variations
  High-voltage applications (>1000V)
  Operation at high junction temperature is allowed (>100 ° C)
  >5kW output power
  
  
  Typical IGBT applications include:
  
  
  Motor control: Frequency &lt;20kHz, short circuit/in-rush limit protection
  Uninterruptible power supply (UPS): Constant load, typically low frequency
  Welding: High average current, low frequency (&lt;50kHz), ZVS circuitry
  Low-power lighting: Low frequency (&lt;100kHz)
  
  
  MOSFETs are preferred in:
  
  
  High frequency applications (>200kHz)
  Wide line or load variations
  Long duty cycles
  Low-voltage applications (&lt;250V)
  &lt; 500W output power
  
  
  Typical MOSFET applications include:
  
  
  Switch mode power supplies (SMPS): Hard switching above 200kHz
  Switch mode power supplies (SMPS): ZVS below 1000 watts
  Battery charging