With the right additional hardware, the rtl-sdr can be used as a super cheap radio telescope for radio astronomy experiments.
Two slightly-different designs for a simple, small, effective, radio telescope capable of observing the Sun, and the galactic plane in both continuum and spectral modes, easily able to show the hydrogen line in various parts of the galactic plane.
He uses the rtl-sdr as the receiving radio with an LNA and a couple of line amps, a 93cm x 85cm offset satellite dish (potential dish for sale here, and here), and GNU Radio with the simple_ra application. In his results he was able to observe the spectrum of the Galactic Plane, and the Hydrogen Line. Some more information about this project can be found on this Reddit thread.
Here is a link to an interesting gif Marcus made with his rtl-sdr, showing a timelapse of recorded hydrogen emissions over 24 hours. Reddit user patchvonbraun (A.K.A Marcus Leech) writes on this thread an explanation of what is going on in the gif.
And here is just one of his many resulting graphs shown in the document showing the Hydrogen line.
A similar radio astronomy project has previously been done with the Funcube. More information about that project can be found in this pdf file. In that project they used the Funcube, a 3 meter satellite dish and the Radio Eyes software.
However, in this Reddit post patchvonbraun explains that the Funcube’s much smaller bandwidth is problematic, and so the rtl-sdr may actually be better suited for radio astronomy.
This image is from the Funcube project document.
Another related project is the Itty Bitty Telescope (IBT), which does not use SDR, but may be of interest.