In short, nsclick is the Click Modular Router embedded inside of the ns-2 network simulator. With minor modifications, a Click routing graph may run both on an actual system as well as under ns-2. This facilitates testing and debugging of network code. For example, a number of ad hoc routing protocols have been implemented and examined under ns-2. This is all well and good, but when time comes to construct a real ad hoc network, the routing code must be redesigned and rewritten. Using nsclick the implementation could be coded in Click, initial testing and debugging performed under ns-2, and that same code, with only minor modifications, deployed on an actual system.
Download the patch release as a tar.gz file here, or take a look at the individual files here. Support occurs via the nsclick-users e-mail list. To sign up, see http://www.cs.colorado.edu/mailman/listinfo/nsclick-users.