** Update**
The 2nd one you posted a link to (Silex) seems to actually work- reviews were more positive.
That keyspan server allows the sharing of USB devices.... kinda.
Printer sharing is no problem, but *ONLY* printers can be automatically accessed on the fly. All other USB devices can only be connected to one computer at a time (just like plugging in a cable) and anyone currently using a device must manually disconnect their system before another computer can access it (unplugging the cable)
So they emulate it through software, but they don't really overcome the basic limitations of USB. It basically eliminates cable length problems. Convenient if devices are spread out over a room or you want to avoid crawling under desks, that's about it. It's also much slower (USB 1.1 speeds are the maximum)
No matter which device you opt for, you'll have to physically connect any USB devices you want to use in the future directly, and install the drivers locally beforehand- on EACH computer.
Bottom line: computers are still the only devices which currently have the flexibility to share "whatever" USB thingy is attached to them, in a dynamic fashion. At some point someone will squeeze a fully functional Windows PC with network sharing onto a Flash drive attached to a USB hub and sell it as a standalone device, but it hasn't happened yet.
Check the reviews!
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There is a USB-over-Ethernet module you can buy, but it still requires a PC driving the USB devices, so I don't see how it's that useful. Maybe it requires less overheard than a shared network device.