Spectrum Waterfall for rtl-sdr written in Java
To get samples you need a RTL2832U and rtl-sdr from http://sdr.osmocom.org/trac/wiki/rtl-sdr
Then simply plug in your RTL2832U and start rtl_tcp. You can then click the start button and see / tune the spectrum.
This is a small tool I wrote because I wanted a java interface into the rtl-sdr and a lightweight way to view the spectrum. I wrote the tool in Netbeans and the repo is a Netbeans project.