I plugged something into an outlet and it stopped working along with another outlet and a lightswitch in an adjoining room... I checked the circuit and nothing had been tripped... what is the most likely culprit and fix for the problem?

If you're sure it's not the circuit breaker - perhaps these two outlets and light are all connected to a comon Ground Fault Interrupt (GFI) device which has tripped. Look to see if any of the outlets have small "Test" and "Push to Reset" buttons in the front face. GFIs are typically used in kitchens, bathrooms, garages, etc - wherever there is risk of exposure to water. Often one GFI is wired to protect several other outlets "downstream" so when it trips multiple outlets can be affected. If you find a GFI outlet, press the "Reset" button and see if that clears the problem.

I can see a popped GFCI but also check the wire connections at each outlet to make sure they are tight. If you used a back stab coonection un do it and use the side screws instead. Test for power coming into each box and then leaving each receptacle. How did you test your breaker to be sure it is not bad?