Need a few hundred machines to run a networking experiment of yours? Often times, you don't need to set up your own, you can use one of the publicly available testbeds below!
PlanetLab over 1000 machines deployed at universities and research labs around the world. Use this testbed to try out large-scale distributed or peer-to-peer applications, or simply to compare the speed of your Internet connection to those of your buddies.
Emulab 160 high-end computers, and another 160+ less powerful machines all connected with multiple network interfaces using high-end switching fabrics. Use this testbed to try out your protocols on diverse topologies, or where low latency is a key constraint.
Orbit Lab Orbit is a 400-node wireless network housed inside a single room. Use this for initial experiments in large-scale wireless networks. Keep in mind that a realistic deployment most likely will run into any number of environmental factors that are not present in this type of 'clean-room' environment.
MoteLab Harvard's MoteLab network is a large sensor-network testbed. Use this to try out your sensor network ideas on low-power, low-cost hardware.