This is a pretty neat new project, which provides 7 channels and integrates LEDs with optical filters in order to produce user-specified digitized versions of arbitrary spectra. For instance, if you use 25nm band filters from 460nm - 635nm in 25nm intervals, you could produce an approximation to just about any spectrum centered in the visible. It is possible to make a larger number of channels, of course, but then spatial mixing starts to suffer without specialized mounting systems.

Here is a tutorial for using the optics, as well as a tutorial for using the prototype controller.