Electrical – Could power fluctuations have killed the refrigerator

Think of the electricity flowing into your house just like water flowing through a pipe. If you turn on a faucet only halfway then you will probably not notice a drop in pressure elsewhere in the house. But if your washing machine is filling (or your irrigation system kicks in, etc.) then you will see reduced pressure elsewhere in the house.

The electricity in your house works the same way. Air conditioners, refrigerators, water pumps, draw a lot of electricity when they first start up and you may see a drop in "pressure" when any of those items turn on:

Air conditioners, refrigerators, freezers, furnaces, etc. that have large motors require enormous amounts of power when the motor first starts to spin up to speed. Once the motor is spinning, the energy requirement drops way down. This instantaneous need for motor shaft speed is what causes that momentary dimming of your lights.
Reference: http://www.askthebuilder.com/QA_Lights_Dim_When_AC_Comes_On.shtml

As @Tester101 mentioned in the comments, you could look into a service upgrade to your house (which might not be a bad idea) but the problem might also be with your water pump. The pump kicking on/off every 15 seconds may indicate a problem with the pump itself (capacitor or pressure leak) or possibly an incorrectly sized pump for your usage. You probably want to look into a new/repair/upgrade of the water pump.

The problem isn't within the toaster, and adding a "power conditioner" wouldn't help. It's simply drawing a lot of power from the outlet, and the wiring impedance between the outlet and the distribution panel is causing the voltage at the outlet to drop a bit.

In general, the lighting circuits and the countertop appliance circuits should be on separate breakers to begin with, so that an appliance fault won't leave you in the dark with a potentially dangerous situation (hot and/or spinning objects).

You should try to work out which outlets in your kitchen are on which circuits, and operate the toaster on a separate circuit from the one your lights are plugged into.