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.
Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.