This arises from off topic comments in another thread. The aim is to expand and hopefully provide searchable data. Data on useful devices would be welcome.
Firstly it is important to appreciate that NO suppression device is 100% effective, the aim is to reduce interference to a negligible level so sometimes both source and modem need attention.
1) Power line spikes have various causes and surprisingly may be more prevalent rural areas. The normal 240 AC mains has a peak of 340v (240 * root 2) but short spikes of several kV can happen.
These were destructive to early semiconductors but should not now be a problem with good equipment design. They may cause noise spikes on xDSL .
They can be suppressed with a solid state equivalent of a spark gap.
2) Current surges are due to equipment design, typically motor startup or initial charging of a capacitor. This may show up as occasional fuse blowing. These can be reduced with a solid state device with a high cold resistance which drops as the device warms up, so smoothing out the peak. Well designed equipment should take care of itself.
3) Continuous interference, from say a TV or a vacuum cleaner, is at low level relative to the 240 v power, but, in addition to suppression for the vacuum, can be reduced because it occupies a different frequency band. A filter has to be designed for it's specific job eg a filter which reduces interference to Long and Medium wave radio will not protect all of the ADSL range. As interference can enter the modem via the power line and/or be radiated from the power line and picked up on the ADSL signal input, it is best to install the filter as close as possible to the interference source.
If it doesn't have one already, a filter at the modem power input will help too.
Devices sold as "mains conditioners" will probably include a spike clipper only. Whilst this isn't useless it is not a fix for xDSL problems.