The rtl-sdr dongle can be been used as a super cheap $20 real-time spectrum analyzer. This is great as commercial spectrum analyzers can run from hundreds to thousands of dollars.
First of all, SDRSharp and other SDR software packages are essentially spectrum analyzers themselves. They all have real time frequency spectrum and waterfall viewers. They are however, not exactly portable.
A portable spectrum analyzer is shown by amateur radio hobbyist and blogger OZ9AEC. He has combined an rtl-sdr dongle, a low cost portable Linux computer known as Beaglebone, and an LCD screen, and turned them into a portable spectrum analyzer. OZ9AEC shows a video demo of his project in this video.
A cheap spectrum analyzer can be useful in many situations. For example, Marek Wodzinski uses the rtl-sdr as a spectrum analyzer for optimizing his FPV radio transmissions. Many other people seem to be interested in a rtl-sdr for this purpose too.
A Google groups post discussing use of the rtl-sdr dongle as a spectrum analyzer is here.