Just found out there is the W5500...
The datasheet says: supports "power down mode" and "Wake on LAN"
I wonder if this is of any use for the Arduino.
Lets say i want to build a LAN controllable RGB lamp...
That means the lamp needs to listen always on the NIC if something is coming in to tell her on/off or change color.
So is it possible to save power with this two functions?
don't know, do you have a library for the 5500?
or at least a link to the datasheet?
Found this in the WIZnet User forum:
If you use the power down mode, the PHY section in W5500 will turn off.
Hence, W5500 can't use WOL function in power down mode.
So seems like there is no way to save power on LAN connected Arduinos 
EDIT:
By looking at the datasheets the have different power consumption on 10M and 100M.
W5100 PDF
W5200 PDF
W5500 PDF
So a W5500 uses a lot less then a W5100 on 10M or 100M.
My questions now are:
If an Arduino can't handle more then 10M then you would save even more power because
the switches also lower the power consumption based on link speed (and cable length).
EDIT #2:
This guy talks about 200 kilobyte on the Arduino with tweaks:
Faster Ethernet Anyone
Thats 1.6 megabit - right?
So 100M does not make sense on the 328/2560 and is power wasting.
MrGlasspoole:
...
My questions now are:
- Can a 328/2560 saturate 10M? Does 100M make sense?
- Is it possible to set the link speed in the code?
...
1.a. No, saturation is beyond UNO/MEGA imho
1.b. Yes, the receiving end might benefit from the faster transmission
2. don't know
As far as I understood the schematic, the WOL/INTn interrupt pin is not brought to an Arduino pin. Still you could solder a wire to export it to an Arduino pin.
How could the other end (switch) benefit if there is not more data coming in?
EDIT:
I ordered some W5500 and now have to wait for the Chinese 
If i can't set the link speed in the code then i guess i can do it in the Zyxel GS1910-24
that i want to buy.
I did some calculation based on max dissipation and 0.25€ per kilowatt.
3.3V x 0.183A = 0.604 x 24 x 365 : 1000 = 5.29kW = 1.32€
10 Devices = 13.20€
20 Devices = 26.40€
3.3V x 0.175A = 0.578 x 24 x 365 : 1000 = 5.06kW = 1.27€
10 Devices = 12.70€
20 Devices = 25.40€
3.3V x 0.125A = 0.413 x 24 x 365 : 1000 = 3.62kW = 0.91€
10 Devices = 9.10€
20 Devices = 18.20€
3.3V x 0.132A = 0.436 x 24 x 365 : 1000 = 3.82kW = 0.96€
10 Devices = 9.60€
20 Devices = 19.20€
3.3V x 0.008A = 0.264 x 24 x 365 : 1000 = 2.32kW = 0.58€
10 Devices = 5.80€
20 Devices = 11.60€
I have the W5500 now and did some measurements.
I just connected power (5V and no Arduino) and switched between 100M and 10M in my Zyxel switch.
W5100 unplugged = 186mA
W5100 @ 100M = 163mA
W5100 @ 10M = 153mA
W5500 unplugged = 74mA
W5500 @ 100M = 136mA
W5500 @ 10M = 81mA
So the W5500 at 10M really saves allot of power.