all the tempest technologies make software back doors look like the stone age.
Actually TEMPEST interception technologies are basically somewhat "stone age" with respect to most name brand computers sold by reputable finished product manufacturers in the US, but there are still many, many exceptions.
Many home-brew PC kit computers produced out of Ma & PA store computer parts from bargain-basement Asian gray market parts importers are, aside from mere 'cheapo' monitors (where passwords are screen-hidden by default custom anyways) notorious for (offering often quite compromising) RF leakage. Some "high performance" and especially overclocked motherboards, thus with mismatched filtering, practically broadcast your data over all landline connections (power, phone, keyboard, network, USB etc). Ever notice those big round bulky ferrite beads on more expensive interconnect cables?
Accessories (not FCC tested) are also a HUGE nightmare.
Ever stopped to think how your wireless keyboard (even if bluetooth "paired") is not encrypted? With a little snooping and forensic work it's childsplay to intercept passwords and forensically or by observation match keystrokes to weblogs, IP requests or even when you power up your "secure" session on your PC or Mac. If you have a wired keyboard it might be a good idea to go to the trouble LOL
There are a lot of other stories, but I thought I'd pass these couple "no-brainers" on.