multimode has a very full and configurable GUI (it works great with GPU accelerated displays like gr-fosphor).
For command line and low power devices try keenerd's rtl_fm.
Antti Palosaari's measurements show the R820T use ~300m A of 5v USB power while the E4000 devices use only ~170m A.
You can cut the leads to the LED to drop usage ~10%.
Originally meant for television reception and streaming the discovery and exploitation of the separate raw mode used in FM reception was perhaps first noticed by Eric Fry in March of 2010 and then expanded upon by Antti Palosaari in Feb 2012 who found that these devices can output unsigned 8bit I/Q samples at high rates. rtlsdr as we know it today was created by the osmocom people in the form of rtl-sdr and osmo SDR, keenerd is the author of many other rtl_* tools: rtl_fm, rtl_power (heatmap.py), rtl_adsb and code changes accepted into the mainline. The dongles with an E4000 tuner can range between 54-2147 MHz (in my experience) with a gap over 1100-1250 MHz in general.
A lot of people other people have helped build it up from there.
Linux 3.x kernel should check with "$ lsmod | grep dvb_usb_rtl28xxu" and if found at least "$ sudo modprobe -r dvb_usb_rtl28xxu" to unload it.
While the sampling bandwidth is only 2.56 MHz the frequency can be re-tuned up to ~40 times a second.
The R820T dongles use a 3.57 MHz or 4.57 MHz intermediate frequency (IF) while the E4000s use a Zero-IF.
The RTL2832 ADC differential input impedance is ~3,300 Ohm.
The dynamic range for most dongles is around 45 d B.
You'll almost certainly notice a stable spike around DC.
It's from either the 1/f noise of the electronics or if it's a Zero-IF tuner (E4000) the LO beating with itself in the mixer.
My favorite way to explore the spectrum is using rtl_power to do very wideband multi-day surveys.