PC's
broadband cable modems
small 120 to 12 V type of wal-warts
Or,... ?

All of the above. But some comments:

Monitors can make a lot of harmonic hash throughout the HF spectrum.
TV's don't seem quite so bad in general (3.58MHz and harmonics are of
course a problem!) and I don't know much about HDTV's etc.

Most wall-warts don't make a lot of noise, but small switching supplies
are an exception.

Fluorescent lamps and dimmers can make an awful racket. Electronic
ballasts make a qualitatively different kind of hash than magnetic
ballasts. Some low-voltage DC lighting and halogen lamps make
incredible noise, I have no idea why (maybe a built-in dimmer?)

Ethernet cabling running around the house can make some, especially
concentrated at 10MHz and harmonics.

Noise can be conducted over and radiated by power cables, ethernet
cables, video cables, keyboard cables, etc.

Motors can always be a source. Kitchen appliances are awful but not
usually run for more than a few minutes at a time. Vacuum cleaners can
be almost as bad. Furnace blowers, AC compressors, refrigerator
compressors aren't as awful but in a modern house at least one of these
is probably running somewhere.

Sometimes I fantasize about moving out to the middle of nowhere, where
there is no power grid!

Tim.