You can run games directly over Ethernet, which I believe can run at close to full speed. Some people have made dedicated little server devices for this out of cheap single-board computers like a Raspberry Pi; I think one guy may even have been selling a finished product like this for a while.

And to be really picky, the first version of the slim actually has IDE HDD support onboard like the original network adapter, just no physical connector (you have to solder one on yourself).