Could you give more specifics about what you are looking for?
If the goal is overall energy or resource efficiency, then using a second-hand switch would probably be the most efficient. (Taking into account the environmental cost of manufacture, shipping, recycling, disposing of, etc.)
There are tons of switches already built - so extending useful life
of a reasonable switch is probably more efficient than creating something new.
If need lowest power draw (e.g. to run off grid), then obviously
avoid more power hungry features like routers and wireless
(unless you need them).
http://www.treehugger.com/files/2007/03 ... m_ener.php
has a link to a database of power usage of some devices,
can't remember if it covers things like net switches, and the database seems to be down at the moment.
You might also check some of the information about power over ethernet
on some of the networking sites. (Might help you find switches that don't take lots of power - although you would still have the question of how efficient the switch's power supply was.)
There have been some threads on SPCR about switching vs. non-switching power bricks, which might help there. Or if you have a computer that will be on whenever the switch is needed, you could run the switch with power from the computer. That would mean the switch was off when not needed, and, assuming the computer has a high efficiency supply, it might use less energy than a separate power brick. It also would not entail the energy of creating yet another power supply (to replace an in-efficient power brick).