EST Approved? I've never heard of an approval body by that name.
These devices will switch off all appliances plugged into them, often they look like an extension cord with multiple outlets. You won't need one in every room in the house, it's generally only AV equipment that have a standby mode. You shouldn't use these for PCs in standby.
There's a lot of mis-information out there in relation to standby power usage as well. While a lot of older or lower end appliances can use significant power in standby mode, modern appliences are pretty efficient. For example, a Panasonic 50" plasma uses 0.4W in standby mode. So it would take 2500 hours in standby mode for this set to use one unit of electricity. A standby saver will save you less than 50 cent a year on that appliance alone.
Leo