Nplate implements in Stata the estimator suggested by Frölich (2007, “Nonparametric IV estimation of local average treatment effects with covariates”).
The easiest way to install the commands is to type
net install nplate, from("http://www.econ.brown.edu/fac/Blaise_Melly/")
on the Stata command prompt.
Alternatively you can download the .ado and .hlp files and copy them in your personal ado directory. (Where is my personal ado directory?)