Using an ULN2803 with Netduino
The ULN2803 is generally meant to work with 5 V it seems, but based on some of the datasheets it might be able to supply 100 mA with 3.3 V in.  The related ULN2805 appears to function better at lower voltages.

If you're looking for a single low-side driver (where the device is always connected to the positive supply while you switch the ground), you can use a logic-level N-channel MOSFET (tutorial).

If you're looking to switch several loads, a high current shift register might be of use to you, and it also can save some I/O pins at the expense of some more complicated driving logic