What are the longest radio wavelengths observed from radio sources in the sky? At what facilities?
The Low-Frequency Array for Radio Astronomy (LOFAR) operates as low as 10 MHz (about 30 meters). You can read more about it here: http://www.lofar.org/
What kind of information does longer radio waves provide over shorter radio waves?
Generally speaking, there are several sources which are brighter at longer wavelengths, like supernova remnants, pulsars, etc. The exact science that one hopes to do with long wavelength observations is extremely technical, a brief summary of which you can find at this website.
This page was last updated July 18, 2015.