SETI@home uses frequency histograms to spot frequencies where we've detected an inordinate number of radio waves.
Such a "spike" of events at a particular frequency is typically representative of either test signals injected into our data recorder (to verify proper functioning) or RFI that uses specific transmission frequencies.
Above are two sample frequency histograms taken from spike data recorded on May 24, 2000. The histogram on the left plots all spikes recorded on that day; the one on the right ignores artificially injected spikes at 1418MHz, 1420Mhz, and 1421Mhz. Also viewable are power histograms from the same day.