Not free but reasonably cheap from DealExtreme:NC120 UXP
- US$45.90NC300 1-to-3 User
Note these are copies of the real NComputing
devices but if you want to try one and don't need USB the NC120 might work out OK.
EDIT: You'll need one Ethernet port per device also but they're reasonably cheap to add, (the NC300 comes with a 3 port card).
NOTE: I'm not recommending you get these from DX, just pointing out they are available. If you want to be sure you get one that works, then buy from NComputing or another brand name company.
A quick look over SourceForge seemed to reveal that there is possibly software available to do the same, (mostly Linux based), but you'd still need to supply your own thin client hardware.